The Chinese government often says Uighur terrorists are to blame for a surge in violence in Xinjiang, while foreign scholars and terrorism analysts say the government appears to be exaggerating its reports of terrorist activities. Beijing has not released much evidence or details of what it has labeled terrorist cells and operations in the region.

The leader of ISIS, Abu Bakr al-Baghdadi, mentioned Chinese fighters in a speech this summer at the start of Ramadan, the Muslim month of fasting. In the nearly 20-minute speech, released online July 1, Mr. Baghdadi listed 12 nationalities of fighters in ISIS, one of them being Chinese, according to a translation by SITE Intelligence Group, an organization in Maryland that tracks jihadist messages.

The recording was the first from Mr. Baghdadi after he had proclaimed himself caliph of the transnational territory of the Islamic State, and in it he encouraged Muslims to engage in jihad. He also called for religious workers, doctors, military experts and others with technical skills to join ISIS.

China was named first in a long list of countries where “Muslims’ rights are forcibly seized.”

There are several reasons jihadists would be hostile to China. Uighurs in Xinjiang have long complained about repressive policies imposed by Beijing. And along with Russia and Iran, China is a prominent ally of the Syrian government, which jihadists are trying to topple.

Raffaello Pantucci, director of international security studies at the Royal United Services Institute in Britain, said there had been reports of Central Asians, Japanese and Chinese fighting in Syria and Iraq. China is worried about the jihadists’ returning home for the same reasons that Western nations are concerned about returning fighters, he said, but the threat could be lower.

“We don’t know where these Chinese are actually from,” he said. “Given the expectation that most Chinese out there will be of Uighur ethnicity, we don’t know that they have all actually come from China rather than from the large diaspora living in Turkey. If they are Turkey-based, then China would not necessarily be their first port of call to return to.”

“It also seems much harder for anyone to get back from Syria to China without attracting attention or falling afoul of border guards in China or elsewhere, given the difficult journey,” he added.

Last year, a video was posted online that showed a Chinese man whom the producers identified as Bo Wang fighting against the Syrian government. The video, which could not be verified, did not say to which group the fighter belonged. The man, who spoke Mandarin Chinese and called himself Yusef in the video, fired a Kalashnikov rifle in a field of flowers. He called on the Chinese government to drop its support of Bashar al-Assad, the ruler of Syria, or “all Islamic countries of the world will unite to impose economic sanctions against the Chinese government.”