Monitoring group says video on internet shows an explosion over a Beijing venue.
Xinhua said the attack happened near Kashgar, a border city where 16 police officers were killed in an attack last week.
China blamed that incident, which targeted a group of 70 police officers as they jogged outside the building, on Uighur separatists.
At least five people were killed and three injured on Sunday in a series of bombings in the town of Kuqa, in western Xinjiang, according to China’s state-run Xinhua news agency.
Dilxat Raxit, spokesman for the German-based World Uighur Congress, said authorities in Kuqa had since detained more than 90 people.
“This includes women,” he wrote in an email, quoting local Uighurs he had talked to by telephone. “They have also been mass detentions in adjacent areas.”
Xinjiang, a vast area that borders Central Asia, has about 8.3 million ethnic Muslim Uighurs, many of whom express anger at what they say has been decades of repressive Chinese rule.
A group calling itself the Turkistan Islamic Party (TIP) released a video last week threatening attacks on China during the Olympic games in Beijing, according to internet monitoring groups in the US.
The video, dated August 1, followed the posting of a similar video in July, in which the group claimed responsibility for bus bombings in Shanghai and Kunming, capital of the southwestern province of Yunnan.
However, Chinese officials dismissed those claims.
The global intelligence analyst group Stratfor said that the Turkistan Islamic Party is another name used by the East Turkistan Islamic Movement (ETIM), an ethnic Uighur group seeking to create an independent state in Xinjiang province.
China has said that a number of plots targeting the Olympics have been foiled and dozens of people, mainly Uighurs, have been detained in Xinjiang, according to state media.
Human rights groups and exiled Uighurs have said that the Chinese government has exaggerated the threat of attack to justify a crackdown in the region. Beijing is also accused of fuelling discontent by encouraging the migration of millions of Han Chinese into the region.