KAYSERI, Turkey—In hindsight, it was a soccer match that kindled Mehmet’s hatred of Chinese rule and set him on the path to exile.

In 2002, Mehmet was at university in Xinjiang, the northwest corner of China that is home to his Uighur ethnic group and the source of a wave of deadly violence in the past two years. He and some other Uighurs decided to support Turkey in the soccer World Cup, he said.

Most Uighurs are Muslim, speak a Turkic language and consider themselves part of a broad family of ethnic Turks.

But students from China’s ethnic Han majority were offended, Mehmet said. A fight erupted, leading university authorities to expel six of his friends.

So began a political awakening that led Mehmet to a prison labor camp in Xinjiang and ultimately to Turkey, following a perilous two-month voyage, mostly without a passport, through Central and Southeast Asia.

Mehmet is among hundreds, possibly thousands, of Uighurs (pronounced WEE-gurs) who have fled China in recent years, often heading for Turkey via Thailand and Malaysia, say Uighur migrants, activists and government officials from countries along that route.

Their flight is presenting China with many of the same fears that have plagued Western nations as they try to prevent their Muslim nationals from being radicalized or trained to fight overseas.

Fearing Uighur separatists are adopting the ideology and tactics of jihadists, China wants to shut down what state media call the “underground railway,” which Beijing says Uighurs are using to join Islamic State in Syria and Iraq or to escape after committing crimes.

China blamed one attack—a mass knifing at a railway station in the southwestern city of Kunming in March that killed 29 civilians—on Xinjiang separatists trying to flee to Southeast Asia. Beijing has often accused Uighur militants of training in Pakistan or Afghanistan, and fears that those fleeing abroad could return to launch fresh attacks or recruit others via the Internet.

Uighur groups like the World Uyghur Congress say they don’t condone such attacks. They and other human-rights groups say China exaggerates risks posed by fleeing Uighurs and that most are escaping brutal policing and systematic discrimination in Xinjiang. Chinese officials deny they discriminate against Uighurs and say Beijing’s policies bring stability to Xinjiang.

Few Uighurs will discuss the issue publicly in Xinjiang, where police surveillance is strong. Those outside China resist speaking openly, fearing deportation or reprisals against relatives back home.

Mehmet said he changed his name to avoid reprisals. A fluent Mandarin speaker in his 30s who once worked for a state-owned company, he said he rejects jihadist ideology but admitted to meeting in Turkey with a pan-Islamist group banned in China and some other countries.

He made no secret of wanting to resist Chinese rule. “If somebody gave me a gun, I would fight,” he said, sitting outside a Uighur activist center in the central-Turkish city of Kayseri. “China only gives us two options—either we must be exactly like them, or we will be destroyed.”

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Over the past year, Beijing has increased pressure on foreign governments to help track Uighur militants, telling some there are roughly 300 Chinese Uighurs fighting with Islamic State in Syria and Iraq, say people briefed on those discussions.

China’s foreign ministry didn’t respond to inquiries for this article. Asked about the figure in December, Chinese foreign-ministry spokesman Qin Gang told a news briefing he had no specifics, but “with international terrorists crossing borders, China urges countries to join forces” to combat terrorism.

Malaysia’s Home Minister, Ahmad Zahid Hamidi, told reporters on Jan. 21 that a Chinese vice minister of public security told him some Chinese fighting with Islamic State had transited Malaysia.

Other governments haven’t publicly corroborated China’s assertions, although the Iraqi defense ministry published a photograph in September purportedly showing a captured Chinese militant. Chinese terrorism experts have cited a speech by Islamic State leader Abu Bakr Al-Baghdadi listing China among countries where Muslim rights had been violated.

The issue is particularly delicate in Turkey, which has granted Chinese Uighurs sanctuary since the 1950s as part of a policy to welcome ethnic Turks from Eurasia. President Recep Tayyip Erdogan once described the Chinese Uighurs’ plight as “genocide” after 2009 ethnic riots in Urumqi.

Turkey has since forged closer commercial and military ties with China. Chinese firms have won big deals to build rail lines in Turkey and launch its intelligence satellite.

Turkish officials say China’s influence hasn’t eroded their commitment to help Uighurs on humanitarian grounds. They also say Turkey is ready to help China’s counterterrorism efforts, just as it cooperates with Western governments to stem the flow of Western jihadists through Turkey to Syria and Iraq.

“It’s a very sensitive issue and public opinion plays a critical role,” said a Turkish official familiar with negotiations on the subject. “We don’t have a specific policy, encouraging Uighurs to come here. Of course, if someone shows up at our doors, we won’t turn them away.”

Asked if there was evidence of Uighurs joining Islamic State through Turkey, the official leafed through a list of countries that provided names of concern. China hadn’t provided any, he said.

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Some Uighurs escape China via an “underground railway” through Southeast Asia to Turkey. Here, families suspected of being Uighurs fleeing China rest after being detained by authorities in Thailand. PHOTO: REUTERS

In recent weeks, though, tensions between China and Turkey have burst into the open. In mid-January, Turkey’s foreign ministry said 10 Turkish nationals faced trial in China on allegations they helped people illegally cross borders or sold travel documents. The ministry didn’t say who was being assisted, but China’s state-run Global Times said they were Uighurs.

That announcement followed demonstrations in Turkey calling on the Ankara government to protect Uighurs in China.

Chinese and Turkish officials have clashed over roughly 300 suspected Uighurs detained in Thailand since March, whom Thai police said they found hiding on a rubber plantation.

Beijing has pressed Thailand to return the suspected Uighurs, who have no identification documents but claim Turkish descent and ask to go to Turkey, say people involved in those discussions. Turkish Foreign Minister Mevlut Cavusoglu in November said publicly that Turkey informed Thailand it wished to take them in.

China’s foreign ministry, responding to a question about Mr. Cavusoglu’s statement, said: “We urge Turkey to immediately stop interfering in the handling of the relevant case” and “not to send mistaken signals to the outside world that connive in, and even support, illegal immigration activities.”

Sek Wannamethee a Thai-foreign-ministry spokesman, said his government knew the Chinese and Turkish positions but needed time to identify the detainees—men, women and children.

China has long controlled foreign travel by Uighurs, some of whom have for decades waged low-level, but often violent, resistance against Chinese rule in their homeland, which they call East Turkestan.

Uighurs fleeing Xinjiang used to cross illegally into Central Asia. That has become harder as China has boosted cooperation with Central Asian nations, Chinese experts and Uighur activists say.

Beijing tightened restrictions following deadly attacks that indicate a change in tactics by separatists, including hitting civilian targets outside Xinjiang and incorporating jihadist imagery.

China in May launched a counterterrorism campaign in Xinjiang,where hundreds have been arrested, 21 executed and 12 more sentenced to death, according to state-media reports. As a result, growing numbers of Uighurs are fleeing to Southeast Asia, say Uighur activists, Chinese experts and Western rights campaigners.

On Jan. 19, Chinese state media said police shot dead two Uighurs and arrested another who “violently resisted arrest” while trying to illegally enter Vietnam. Rights groups and Uighur activists say it is possible that some attack participants escape this way but that most migrants appear to be fleeing the violence or China’s response.

Many obtain Turkish travel documents—fake or legal—in Thailand or Malaysia, say Uighur migrants and officials along the route.

In September, Indonesian authorities said they arrested four men believed to be Uighurs with fake Turkish passports allegedly trying to link with an Islamic militant leader. The men’s lawyer said they are in detention in Indonesia and deny all charges.

Some Southeast Asian nations have deported Uighurs to China. Cambodia announced deportations in 2009 of Uighurs it said entered illegally. Malaysian police say they have deported to China at least 18 suspected Uighur militants.

“Most of the individuals we know about who have been sent back from a wide variety of countries in recent years have effectively, alarmingly simply vanished into the notoriously abusive vortex of Xinjiang’s judicial system,” said Sophie Richardson, China Director of Human Rights Watch.

China’s foreign ministry has said in news conferences that Uighur deportees to China were criminal suspects.

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Ethnic Uighurs like these men in Kayseri, Turkey, consider themselves part of a broad family of Turkic peoples and use the name East Turkestan (Dogu Turkistan in Turkish) for the mostly Muslim minority’s homeland in China’s Xinjiang region.PHOTO: JEREMY PAGE/THE WALL STREET JOURNAL

Those who make it to Turkey often settle in Uighur communities in Istanbul’s Zeytinburnu neighborhood or in Kayseri, where Turkish authorities settled a group of Chinese Uighurs who escaped via Afghanistan in 1965.

An elderly Uighur in Zeytinburnu said that after years in prison in Xinjiang for separatist activities, he escaped via Myanmar, Laos, Thailand and Malaysia in 2013. He flew to Turkey after obtaining a Turkish travel document in Malaysia for about 5,000 yuan ($800).

Some Uighurs—mostly women—reach Turkey legally after receiving passports in China. A Uighur in Kayseri said she flew from China in 2014 to seek fertility treatment but dared not return, as her husband was detained in Xinjiang when he applied for a passport to join her.

Mehmet, who reached Kayseri, said that after the 2002 soccer-match fight he began spending more time looking at websites about Uighur issues.

After graduation, he joined a state-owned company—a coveted job in Xinjiang—but became disillusioned that few Uighurs were employed there. And he resented pressure to drink with prospective business partners, because Islam forbids alcohol consumption.

By the time of the 2009 Urumqi riots, he felt such pent-up anger that he joined the violence. He wouldn’t say what he did but said he was jailed for three years.

He shared a prison cell with ethnic-Han members of Falun Gong, the spiritual group banned in China. “There, I realized that not all Han people were bad, and that many of them suffer in the same ways that Uighurs do,” concluding his only option was to flee China.

In 2013, after prison, he borrowed from friends and family to fund his escape. He had no passport, so friends smuggled him into Kyrgyzstan. From there, he said, a people-smuggling network took him through Tajikistan, Afghanistan, Pakistan, India, Bangladesh, Myanmar, Thailand and Malaysia.

He crossed borders in vehicles, on foot and by boat over nine weeks. “Sometimes we had to run across borders at night, sometimes high in the mountains.”

In Malaysia, he obtained a Turkish travel document via an ethnic-Chinese agent. He didn’t know if it was genuine, but it got him into Turkey. He spent some 90,000 yuan (just under $15,000), paying a different person for each leg.

He said he rents a room with another Uighur and does odd jobs in a restaurant while considering his next move. He said he doesn’t believe in jihad but could understand why some do. “Uighurs are looking for a savior,” he said.

In Turkey, he said, he attended a meeting of Hizb ut-Tahrir, the pan-Islamic group banned in China. He left disillusioned after they said they couldn’t provide arms to Uighur separatists.

Hizb ut-Tahrir advocates an Islamic caliphate but only sanctions peaceful means, said Mahmut Kar, the organization’s spokesman in Turkey. The group is active in Xinjiang, he said, but “we have not encountered Uighur Muslims who took refuge in Turkey and sought support for an armed struggle against China, or to go to Syria and Iraq.”

Mehmet said he has spoken online with Uighurs in Turkey who want to join Islamic State but said he wants to head to Europe to work for the Uighur cause.

“Why would I risk my life fighting in Syria or Iraq?” he asked. “If I am going to fight, I want to fight for East Turkestan.”

—Celine Fernandez in Kuala Lumpur, Warangkana Chomchuen in Bangkok and I Made Sentana in Jakarta contributed to this article.

Write to Jeremy Page at jeremy.page@wsj.com and Emre Peker atemre.peker@wsj.com

http://www.wsj.com/articles/as-muslim-uighurs-flee-china-sees-jihad-risk-1422666280