A radical Islamist group has claimed responsibility for an attack on Tiananmen Square last month and warned of future attacks in the Chinese capital, according to an eight-minute audio clip obtained by a US-based internet monitoring organisation.

The Turkistan Islamic Party (TIP) is the first group to claim responsibility for the attack on 28 October, when a four-wheel drive vehicle ploughed through a group of pedestrians near the iconic square in central Beijing, crashed into a stone bridge and caught fire, killing five people and injuring dozens. Chinese authorities quickly identified the driver as Uighur, a Muslim ethnic minority hailing from Xinjiang, a sparsely populated, restive region in the country's far north-west.

"O Chinese unbelievers, know that you have been fooling East Turkistan for the last sixty years, but now they have awakened," the organisation's leader Abdullah Mansour said in the clip, which was posted online this weekend by the Search for International Terrorist Entities Institute (SITE), a Bethesda, Maryland-based website which monitors jihadist forums. Uighur separatists call the region East Turkistan.

Mansour warned of future attacks by Uighur fighters, including one targeting the Great Hall of the People, a granite edifice flanking Tiananmen Square where the ruling Communist party holds many of its highest-level meetings. "The people have learned who is the real enemy and they returned to their religion," he said. "They learned the lesson."

Chinese authorities have blamed the Tiananmen Square attack on the East Turkestan Islamic Movement (ETIM), a shadowy Xinjiang-based group with ostensible ties to al-Qaida.

On Monday, foreign ministry spokesman Qin Gang said ETIM was the same as the TIP, and said the government would "continue the assault" on the group.

"This lays bare the terrorist essence of this organisation and it also allows those people who recently suspected the nature of the incident to clearly see the truth," Qin told a regular press briefing.

But many Xinjiang experts responded with scepticism. They say that the attack was probably motivated by China's hardline regional policies, which place severe restrictions on religious practice. Some doubt that ETIM is organised enough to carry out a sophisticated terrorist attack.

Nicholas Bequelin, a senior researcher at Human Rights Watch in Hong Kong, said that ETIM has reformed as the TIP in recent years; in 2012, its leader was killed by a CIA drone strike in Pakistan. The US department of state labelled ETIM a terrorist organisation in 2002, he said, and the Chinese government continues to use the appellation to lend international credibility to its anti-terrorism programme.

The TIP has claimed responsibility for bus bombings in the Chinese cities Kunming and Shanghai, and threatened attacks on the 2008 Olympics in Beijing. "The problem is that their credibility is dodgy at best," Bequelin said in an interview earlier this month.

"It's not clear whether they are a real outfit that is actively planning things, or just a bunch of people who are sort of agitating."

On Monday, the state-controlled magazine Oriental Outlook reported that Xinjiang experienced 190 "violent terrorist" incidents in 2012 – a "substantial" increase over 2011, it said, citing Xinjiang public security statistics. Because authorities severely restrict the flow of information in Xinjiang, the details of most cases remain murky.

The TIP's statement may allow the authorities to "point to the international community and say yes, we have a serious jihadist problem in Xinjiang", said Michael Clarke, a Xinjiang expert at Griffith University in Australia. They could use it "as an attempt to justify China's hard line" in the region.