China insisted it would be able to hold a safe Olympics after officials said they had foiled two terrorist plots, while analysts and activists expressed scepticism about the extent of any threat.

Wang Lequan, Communist Party boss in Xinjiang, where the largely Muslim, Uighur minority has agitated for greater autonomy and rights, said police had shot dead two members of a "terrorist gang" in a January raid and rounded up 15 others whose aim was to disrupt the Games.

Other officials from the far-northwestern region said a passenger jet bound for Beijing from Xinjiang was diverted on Friday after the discovery of what the state-run Xinhua news agency called a "planned terrorist attack".

"From the very beginning we have attached great importance to Olympic security," said Sun Weide, a spokesman for Beijing's Olympic Organising Committee. "We are confident that we will be able to have a safe Olympic Games."

Rights groups have accused the Chinese government of exaggerating the threat of violence in Xinjiang, which borders Central Asia, in the interests of exerting greater control in the area where China is trying to curb separatist sentiment.

"Anything that is an expression of Uighur identity becomes equated with separatism, which then becomes equated with violent terrorism," said Corinna-Barbara Francis, a China researcher at Amnesty International in London.

Uighurs have been jailed in China on terrorism charges for what in the West would be considered freedom of expression, Francis added.

Rebiya Kadeer, jailed for more than five years for championing the rights of Uighurs before she was sent into exile in the United States, said she did not believe the government's version of the alleged militant activities in Xinjiang.

"This is something they have fabricated themselves, it's an incident they've arranged," she told Reuters by telephone from her home in Washington D.C.

"Right now, Uighur people don't have the ability to do that. Moreover, they don't have any plans to do so," Kadeer said of the plane incident.

Dilxat Raxit, a spokesman for the exiled World Uighur Congress, said China was using the upcoming Olympics as an excuse to crack down further on his people.

"China's aim is to show the world that the Uighurs are terrorists," he said. "We oppose terrorism."

The China Southern flight was en route from Xinjiang's capital, Urumqi, to Beijing on Friday when it made an emergency landing in Lanzhou after the crew discovered what one source said was inflammable material in the toilet.

None of the passengers or crew were harmed, according to the Xinjiang government, and the plane finally arrived in Beijing on Saturday.

Analysts said reporting by China on politically motivated plots should be treated with caution.

"I note in possible support of China's claim, that Uighur separatists do seem to be trying to involve foreigners in incidents to make it harder for the government to hush them up," said Clive Williams, a lecturer on terrorism at Australia's Macquarie University.

But, in an e-mailed response to questions, he added,

"China's claim could be intended to set the scene for a crackdown on the separatist movement ETIM before the Olympics".

The East Turkestan Independence Movement advocates a separate state for Xinjiang.

A source told Reuters that police had detained two people from the flight, but state media and the government have given no details about the suspects or what their aim might have been.

Travellers flying through Beijing and Shanghai at the weekend reported no heightened security measures.

Reuters