ISIL fighters from China's Uighur ethnic minority have vowed to return home and "shed blood like rivers" in what security experts said marked the first such threat against Chinese targets.

The threat came in a half-hour video released Monday by a division of Islamic State of Iraq and the Levant (ISIL) featuring fighters from China's Uighur minority, said the US-based SITE Intelligence Group, which analysed the footage.

China has for years blamed exiled Uighur "separatists" for a series of violent attacks in its western Xinjiang region - the Muslim Uighur homeland - and warned of the potential for them to link up with violent groups.

In the video, a Uighur fighter issued the threat against China just before executing an alleged informant.

"Oh, you Chinese who do not understand what people say. We are the soldiers of the Caliphate, and we will come to you to clarify to you with the tongues of our weapons, to shed blood like rivers and avenging the oppressed," according to SITE's translation.

The footage also showed fighters, including heavily armed children, giving speeches, praying, and killing other "informants".

Many Muslim Uighurs complain of cultural and religious repression and discrimination by China.

Authorities have banned or strictly controlled the observance of certain Muslim practices, such as growing beards, wearing headscarves, and fasting during Ramadan, calling them symbols of "Islamic extremism".

A US think-tank said in July that Chinese religious restrictions on Muslims may have driven more than 100 to join ISIL.

"When we see the government involved in a very heavy crackdown, it hasn't really ever solved the problem, it hasn't made it go away," said Raffaello Pantucci, director of International Security Studies at the UK-based Royal United Services Institute.

"In some cases, it has made it worse."

READ MORE: China targets parents with religion rules in Xinjiang

The video appears to be ISIL's "first direct threat" against China, Michael Clarke, an expert on Xinjiang at the National Security College of Australian National University, told AFP news agency.

"It is the first time that Uighur-speaking militants have claimed allegiance to IS," he added.

The video showed China is now "very firmly a target of jihadist rhetoric", Clarke said, marking a shift from years past when it rarely figured in statements by armed groups.

But Clarke said it also could indicate a possible split among Uighur fighters, as it includes a warning to those fighting with the al-Qaeda-aligned Turkistan Islamic Party (TIP) in Syria.

China's foreign ministry spokesman Geng Shuang said Wednesday he had not seen the video but noted that "East Turkestan terrorist forces have been posing a severe threat to China's security."

He called for international cooperation "to combat such terrorist forces".

Show of force

China maintains tight security in Xinjiang but a drumbeat of deadly unrest has continued. A knife attack last month left eight dead, including three attackers, police said.

The video was released on the same day China staged the latest in a series of mass rallies by armed police in Xinjiang meant to indicate Chinese resolve in crushing security threats.

More than 10,000 officers gathered Monday in the regional capital, Urumqi - the fourth such show of force this year in Xinjiang.

In one violence-wracked corner of Xinjiang, authorities are offering rewards of up to $730,000 to those who expose attack plots or "kill, wound, or subdue" any assailants.

The ISIL video also featured images of Chinese riot police guarding mosques, patrolling Uighur markets, and arresting men in what appears to be western China. The Chinese flag is pictured engulfed in flames.

Clarke said the hints of a Uighur split could "intensify the threat to China" as it indicates Uighur fighters may be able to tap into the capabilities of both ISIL and al-Qaeda.

Overseas analysts have up to now expressed doubts about the strength of Uighur fighters, with some saying China exaggerates the threat to justify tough security.