China reported the highest number of asymptomatic novel coronavirus infections in nearly seven months following the discovery of a cluster of cases linked to a garment factory in the northwestern region of Xinjiang.

Health authorities found 137 asymptomatic cases on Sunday during a drive to test 4.75 million people in the Kashgar area triggered by an asymptomatic infection in a 17-year-old female garment factory worker reported on Saturday.

It was not clear how the teenager was infected though the official Xinhua news agency said all of the new cases were linked to another garment factory where the patient's parents work. The parents had not tested positive for the virus, however, it said.

The teenager's infection was discovered during routine weekly testing in Xinjiang for what Xinhua called key groups of people. It did not specify who was in those groups or why she was among those tested, Reuters reported.