In the video, a Uighur fighter issued the threat against China just before executing an alleged informant.
Islamic State militants from China's Uighur ethnic minority have vowed to return home and "shed blood like rivers", according to a jihadist-tracking firm, in what experts said marked the first IS threat against Chinese targets.
The threat came in a half-hour video released Monday by a division of the Islamic State in western Iraq and featuring militants from China's Uighur ethnic group, 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 Uighur homeland -- and warned of the potential for militants to link up with global jihadist 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.
A traditionally Muslim group, many Uighurs complain of cultural and religious repression and discrimination by China.
It appears to be the Islamic State's "first direct threat" against China, Dr. Michael Clarke, an expert on Xinjiang at the National Security College of Australian National University, told AFP.
"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 global jihadist 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.
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 the same day that China held the latest in a series of mass rallies of military police in Xinjiang meant to indicate Chinese resolve in crushing security threats.
More than 10,000 officers gathered Monday in the region's capital Urumqi -- the fourth such rally this year in Xinjiang.
Chinese authorities have tightened controls in the region, beefing up police checkpoints.
In one violence-wracked corner of Xinjiang, authorities are offering rewards of up to 5 million yuan ($730,000) to those who expose terror plots or "struggle, kill, wound, or subdue" any attackers.
The Islamic State video showed fighters, including heavily armed children, giving speeches, praying, and killing other "informants".
It 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 militants may be able to tap into the capabilities of both the Islamic State and al-Qaeda.
Overseas experts have up to now expressed doubts about the strength of Uighur militants, with some saying China exaggerates the threat to justify tough security measures.
A US think tank said in July that tough Chinese religious restrictions on Muslims may have driven more than 100 to join the Islamic State.
Authorities have banned or strictly controlled the observance of certain Muslim practices, such as growing beards, wearing headscarves, and fasting during Ramadan, saying they were symbols of “Islamic extremism”.
