Chinese Internet users on Monday accused the US of double standards after Washington condemned a deadly knife attack in southwest China but refrained from calling it a terrorist incident.

The US embassy in China said on social media that it condemned the "terrible and senseless act of violence in Kunming" and expressed condolences to those affected in what it said was a "tragedy".

The attack on Saturday night saw masked assailants stab civilians at a train station, killing 29 and wounding more than 130.

China has blamed separatists from the restive region of Xinjiang for what it described as an act of terror, with state media dubbing the incident "China's 9/11".

But thousands of Chinese Internet users slammed the US for refusing to follow China in defining the attack as terrorism, comparing the knifings to last year's bombing of the Boston Marathon as well as 9/11.

"Merely 'terrible and senseless'? Only 'violence'? Would Americans say the same thing about similar attacks on their own territory?" Ma Xiaolin, a website administrator, asked on Sina Weibo, a Chinese equivalent of Twitter.

"If you say that the Kunming attack is a 'terrible and senseless act of violence', then the 9/11 attack can be called a 'regrettable traffic incident'," posted Cao Fan.

In typical mocking response, another user wrote: "I express my condolences for the setting off of fireworks and burning incident at the Boston Marathon."

The US statement came after the UN Security Council condemned the killings and said that it "underlined the need to bring perpetrators, organisers, financiers and sponsors of this terrorist attack to justice".

It added: "Any acts of terrorism are criminal and unjustifiable regardless of their motivation."

Russian President Vladimir Putin also described the attack as "terror" in a message sent to China's President Xi Jinping, state media reported on Monday.

Following the September 11, 2001 attacks on New York and Washington, the US placed the East Turkestan Islamic Movement, which China blames for unrest in the vast Xinjiang region, on its official list of international terrorist organisations.

But the group's strength and links to global terrorism are murky, and some experts say China exaggerates its threat to justify tough security measures in Xinjiang, home to the mainly Muslim Uighur ethnic minority.

Several Uighurs were detained in the US' Guantanamo Bay facility, although nearly all were later released without charge.

But the US has refrained from labelling recent clashes between Uighurs and security forces in Xinjiang as terrorism, while condemning China's treatment of the minority, prompting the foreign ministry in Beijing to repeatedly accuse Washington of a "double standard".

Chinese social media services are subject to strict censorship, with posts and responses critical of China's official stance regularly deleted.