A militant Islamist group has voiced support for a mass stabbing at a Chinese railway station that killed 29 people, the US-based SITE Intelligence Group, which monitors extremist organisations, said Tuesday.

The Turkestan Islamic Party (TIP) released a video online about the March 1 attack at Kunming railway station in the southwestern province of Yunnan.

It described the assault as an "expensive offer" to Beijing to reconsider its "cruel" policies in Xinjiang, the far western Chinese region home to the mostly Muslim Uighur minority, SITE said.

In one segment, an official read a message that the attackers allegedly addressed to Chinese President Xi Jinping, SITE said, but it was not clear whether the video included an explicit claim of responsibility.

According to SITE, the video included audio by TIP leader Abdullah Mansour who said: "If the fighters of East Turkestan are now fighting with swords, knives, and mallets, our dear Allah will soon give us opportunities to fight the Chinese using automatic guns."

East Turkestan is an alternative name for Xinjiang, which has cultural ties to Central Asia.

Four of the attackers were shot dead at the scene, one injured and detained and three more suspects arrested, Chinese authorities said.

"Know that blood of those who are killing themselves is not being spilled for nothing, for their blood will bring tens of more to carry out jihad," SITE quoted Mansour as saying.

SITE said in November that in a video posted online by Mansour described a fiery vehicle crash on Beijing's Tiananmen Square last October as a "jihadi operation" and the perpetrators as "mujahideen".

Beijing regularly accuses what it says are exiled Uighur separatist groups such as the East Turkestan Islamic Movement (ETIM) and TIP as being behind terrorism.

But many outside experts doubt the strength of the groups and their links to global terrorism.

Some argue that China exaggerates the threat to justify tough security measures in Xinjiang, where rights groups say Uighurs are subject to repression.

The shadowy nature of the alleged Uighur extremists also means there is a lack of hard information on their numbers, location and capabilities.