Vowing to plant their flag in China and that blood will "flow in rivers", a new video released this week purportedly by the Islamic State group shows ethnic Uighur fighters training in Iraq, underscoring what Beijing sees as a serious threat.

China is worried that Uighurs, a mostly Muslim people from western China's Xinjiang region, have gone to Syria and Iraq to fight for militant groups there, having travelled illegally via Southeast Asia and Turkey.

Islamic State claimed responsibility for the killing of a Chinese hostage in 2015, demonstrating China's concern about Uighurs it says are fighting in the Middle East.

Hundreds of people have been killed in Xinjiang in the past few years, most in unrest between Uighurs and the ethnic majority Han Chinese. The government blames the unrest on Islamist militants.

The Iraqi arm of Islamic State has released a half-hour long video purportedly showing Uighurs training, as well as some images from inside Xinjiang, including Chinese police on the streets.

One shot that shows Chinese President Xi Jinping gives way to flames in front of a Chinese flag.

"Hey, brothers! Today, we are fighting with infidels across the world! I'm telling you this: 'Come and live here! Stay strong!'," one of the fighters says, according to Uighur speakers who analysed the video for Reuters but who asked not to be identified.

"We will certainly plant our flag over America, China, Russia, and all the infidels of the world," it says.

In another scene, a chant in Uighur says: "Our land of sharia has been constructed with spilt blood." It then shows pictures of martyrs indicated as "al-Turkistānī", or men from Turkestan, the name many Uighurs use for Xinjiang.

One of the men speaking has an accent from Yarkand, close to the old Silk Road city of Kashgar in Xinjiang's southern Uighur heartland, one of the people who reviewed the video said.

Another fighter refers to the "evil Chinese Communist infidel lackeys".

"In retaliation for the tears that flow from the eyes of the oppressed, we will make your blood flow in rivers, by the will of God!" he says.

The video was released this week by the U.S.-based SITE Intelligence Group, which monitors militant groups online.Reuters was not able to independently verify the authenticity of the video.

Chinese Foreign Ministry spokesman Geng Shuang said on Wednesday he was not aware of the video and had not seen it.

"But one point is very clear. We oppose any form of terrorism and proactively participate in international cooperation to crack down on terrorism," Geng told a daily news briefing.

"We have long said that East Turkestan forces are a serious threat to China's security and we are willing to work with the international community to jointly crack down on East Turkestan separatist and terrorist forces," he said.

The government says foreign militants have stirred up tensions in Xinjiang, where it says it faces a determined campaign by separatists who want to establish an independent state called East Turkestan.

However, many rights groups and exiles doubt the existence of a coherent militant group in Xinjiang and say Uighur anger at repressive Chinese policies is more to blame for the unrest.

China denies any repression in Xinjiang.