A brazen and bloody machete attack at a Chinese railway station could herald an ominous change in the violence Beijing blames on separatists from Xinjiang, analysts say, threatening more incidents far beyond the restive region.

Until now, disturbances have largely been confined to Xinjiang, a vast resource-rich area in far western China, and focused on symbols of the Chinese state, such as police.

But the scale, location, nature and timing of the Kunming mayhem potentially points to a change in mindset with the overt targeting of civilians -- whoever was behind it.

Beijing immediately labelled the incident as terrorism, and Washington followed suit on Monday after initially referring to it as a "tragedy", in language that drew angry accusations of double standards on Chinese social media.

"You can't class this attack as other than a terrorist attack," Michael Clarke, an expert on Xinjiang at the Griffith Asia Institute in Brisbane, Australia, told AFP, adding that it was designed to "induce fear amongst the Chinese populace".

Xinjiang is home to Uighurs and other ethnic minorities with strong cultural and historical links to neighbouring Central Asian states such as Uzbekistan and Kazakhstan. It has seen regular clashes for much of the past year.

Uighurs resent decades of immigration by China's Han majority they say has brought economic inequality and discrimination in education and religious affairs, such as a campaign to stop the Muslim practice of women covering their faces.

China counters that it plays a positive role and has brought about development and improvements to health and living standards.

- Rare brutality -

Deadly incidents as reported in state media and by exile groups often centre on violence between Uighurs and local police and security authorities, with both sides trading accusations over who is to blame.

But Saturday's bloodshed in the southwestern city of Kunming, where knife- and machete-wielding attackers clad in black killed 29 people and wounded 143, stands out for its brutality and targeting, analysts say.

"It must have taken a lot of planning and involved the recruitment of a large number of attackers who assumed most would die," said Gardner Bovingdon, an expert on the history and politics of Xinjiang at Indiana University in the United States.

Police captured three suspects on Monday, the ministry of public security said, adding that eight people were involved in all, four shot dead and a wounded woman held at the scene.

State media were quick to point the finger at "Xinjiang separatists", code for Uighurs, the largest single group in Xinjiang.

Some analysts outside mainland China, however, are wary of such assertions until hard evidence emerges, given the state's overwhelming control of domestic information and media.

Clarke said that if eventually proven, it "really signals a new level in development in terms of the conflict there, in a sense spreading the struggle from Xinjiang basically to the rest of China.

"And that has to be concerning for the Communist Party moving forward."

- 'Natural progression of violence' -

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

The US-based SITE Intelligence Group, which monitors extremist groups, said in November that in a video posted online TIP leader Abdullah Mansour described a fiery vehicle crash on Beijing's Tiananmen Square as a "jihadi operation" and the perpetrators "mujahideen".

He warned Uighur fighters would target the Great Hall of the People on Tiananmen Square, where the ruling Communist Party holds key meetings such as this week's annual session of China's rubber-stamp legislature.

"The Kunming attack is either ETIM-led or ETIM-inspired," said Rohan Gunaratna, professor of security studies at Nanyang Technological University in Singapore, describing it as the military wing of the TIP and the only group with the ability to conduct or instigate such an assault.

But other experts doubt the strength of the groups and their links to global terrorism, with some arguing China exaggerates the threat to justify tough security measures in Xinjiang.

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

"I remain unconvinced that ETIM has survived as an influential organisation these last years," Bovingdon said. "Beijing constantly adverts to it precisely because the US once identified ETIM as a terrorist organisation."

He also expressed doubts that the TIP, seen by some as ETIM's successor, has any influence.

"All the evidence I've seen thus far tells us no more than that some bearded Uighur speakers armed with weapons and a smattering of Islamic learning can make videos," he said.

Chung Chien-peng, professor of political science at Lingnan University in Hong Kong, cautioned it was too early to say who was responsible for Kunming.

But it had "shock value", he said, and "desperation or... frustration" may prove to be the cause.

The latest assault will not be the last, Gunaratna believes.

"The Kunming attack is a natural progression of violence that we have witnessed in Xinjiang," he said.

"It's a low-cost, high-impact attack and a long-range attack. And this kind of attack sends a very clear message to China. They will strike again, they will strike outside Xinjiang."