Intelligence reports show that the Eastern Turkistan Islamic Movement (ETIM) has been planning to carry out terrorist attacks during the Games, Director of the Security Command of the Beijing Games Ma Zhenchuan said.
"It is not imaginary. We have been focusing on the group and it has been labelled as a terrorist group not only by our country but also the international community," he said.
China recently said it had smashed five terrorist groups plotting attacks on Olympics and arrested 82 terrorists linked to them in the first six months of this year.
Chinese police had also claimed to have destroyed 41 Islamic military training bases from January to June and busted 12 terrorist cells of overseas-based outfits in the restive Muslim-populated Xinjiang region.
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"The command had already worked out detailed counter terrorism plans," Ma was quoted as saying in an interview to CCTV by the state-run China Daily.
The UN listed the "East Turkistan Islamic Movement" as a terrorist group in 2002 with alleged links to al Qaeda.
The World Uyghu Congress, however, denies it and accuses China of religious repression.
Xinjiang is home to Muslim, Turkic-speaking Uighurs and Beijing had accused militant Uighurs of working with al Qaeda and other groups for an independent state called "East Turkistan."
Ma, however, said ETIM was only part of the terrorist threat to the Olympics. "We have been cooperating with security authorities of all participating countries to thwart any plot to disrupt the Games", he said.