Beijing Olympic organisers sought Monday to play down security concerns looming over the Games, a day after authorities said two "terrorist" plots from its Muslim-majority northwest had been foiled.

"We are confident that we will be able to have a safe Olympics," said Sun Weide, a spokesman from Beijing's Olympic Organising Committee (BOCOG).

Sun gave no indication as to whether any new measures had been put in place since the announcement on Sunday of the apparent terror threats.

But efforts from Beijing to quell fears came amid accusations from exiled Uighur leader Rebiya Kadeer that China fabricated the alleged plots and even schemed to carry out its own terror attacks to blacken her community's name.

Wang Lequan, Communist Party chief in the northwestern Xinjiang region, said Sunday that a January raid on "terrorists," which resulted in the deaths of two militants and 15 arrests, had foiled a planned attack directed at the Games.

It was the first specific threat against this summer's Olympics to be reported by authorities, although Chinese officials had previously warned that terrorism was the biggest threat to the Games.

The alleged plot was the second foiled attack linked to Muslim separatists in Xinjiang to be announced over the weekend.

Passengers on a China Southern Airlines flight attempted to crash a Chinese airliner on Friday flying to Beijing from Urumqi, capital of the region, an official from the region said on Sunday.

As in the first case, few details have emerged except for a brief statement on Monday from the national aviation authority that passengers on the flight had been carrying "suspicious liquids."

The plane was subsequently diverted to the city of Lanzhou in Gansu province, where the substances were removed, the Civil Aviation Administration of China said.

The chairman of the Xinjiang regional government said on Sunday the plane plot was carried out by people "attempting to create an air disaster."

Lanzhou police and Communist party authorities refused to comment on the incident when contacted by AFP on Monday.

The official version of events was disputed by Kadeer, head of the Uyghur American Association.

"It's completely untrue. All these allegations are falsified," Kadeer, who joined her US-based husband in 2005 after six years in a Chinese jail, told AFP through an interpreter.

"The real goal of the Chinese government is to organise a terrorist attack so that it can increase its crackdown on the Uighur people," the 61-year-old said.

China regularly accuses the independence-minded East Turkestan Islamic Movement (ETIM), listed by the United Nations and the United States as a terrorist organisation, of being the most significant threat in Xinjiang.

The majority of the population in Xinjiang, which borders Afghanistan and central Asia, are Muslim Turkic-speaking Uighurs, many of whom bridle at what they say has been 60 years of repressive communist Chinese rule.

Exiled Uighurs such as Kadeer and some prominent Western rights groups say there is no major terrorism threat in Xinjiang, and the battle against "terror" is used as an excuse to silence any voices of dissent, no matter how small.

This may be the case on this occasion, particularly with China worried about any critical voices tarnishing its image before the Olympics, Phelim Kine, Asia researcher with New York-based Human Rights Watch, told AFP.

"We're concerned that the Chinese government may use these alleged terrorist plots as a pretext for a new campaign of repression against the Uighur population in Xinjiang and to stifle any public expressions of dissent in Xinjiang ahead of the 2008 Olympic Games," he said.