Xinjiang province, where it is believed more than one million Uighurs and other Muslims are being held in a vast network of camps, reported 23 new confirmed cases Thursday, all involving people who had initially tested positive but displayed no symptoms.
It was the second consecutive day in which newly confirmed cases emerged.
Officials say that development appears to show new infections have been curbed in Kashgar prefecture, where the outbreak appeared Saturday. They say all the cases seem to be linked to a garment factory that employs 252 people and has since being sealed off.
Chinese officials have been racing to smother the fresh coronavirus outbreak in the country's far northwest after 137 new infections were discovered on the weekend.
Mass testing began Saturday evening to cover 4.75 million residents in and around Kashgar, Xinjiang province, after a 17-year-old garment factory worker tested positive for the virus.
China - where the coronavirus first emerged late last year - has largely brought domestic transmission under control through lockdowns, travel restrictions and testing, but sporadic regional outbreaks have emerged.
Beijing has lauded its rapid testing capabilities, with the Communist Party eager to project an image of victory over the virus as much of the world struggles with lockdowns and mass outbreaks.
The new cases - all asymptomatic - were linked to a factory in Shufu county where the girl and her parents worked, the Xinjiang health commission told a press briefing Sunday.
Senior lecturer in politics and international relations at Australia's James Cook University Dr Anna Hayes told The Guardian that given the evidence of forced labour in the province, the girls' underage status was supicious.
Hayes said she took the reported 137 asymptomatic cases "with a grain of salt", believing China may want to keep news of a large outbreak hidden to avoid further scrutiny of their persecution of Uighurs in the region.
A special team from Beijing's National Health Commission was sent to investigate the source of the outbreak and assist with preventive measures, state broadcaster CCTV reported.
As of Sunday afternoon more than 2.8 million samples had been collected in the area and the rest would be completed within two days, the city government said in a statement.
Kashgar - near the country's borders with Pakistan, Afghanistan, Tajikistan and Kyrgyzstan - is the cultural heart of ethnic Uighurs and other Turkic Muslims, many of whom complain of long-running political and religious oppression, which the Chinese government denies.
Human rights groups have complained of extreme human rights violations against Uighurs by Beijing.
All schools in Kashgar have been closed until October 30 and anyone leaving the city needs to show a negative nucleic acid test, the city government said.
State broadcaster CCTV showed footage of hundreds of people lining up to take nucleic acid tests outside hospitals and mobile testing centres set up across the city.
Urumqi, the capital of Xinjiang, was kept under strict lockdown for weeks after more than 900 cases were reported in mid-July.
Meanwhile, the US envoy for women's issues said Thursday that the United Nations is not doing enough to investigate reported abuses in China's Xinjiang region against members of Muslim minority groups.
Citing reports of forced birth control, home visits and sexual violence in detention centers, Ambassador-at-Large on Women's Issues Kelley Currie said such practices show a "pervasive pattern of targeting women".
The Associated Press has previously reported that China has been carrying out a draconian campaign to cut birth rates among its Uighur Muslim population by forced sterilisation and compulsory family planning practices.
Pointing to what it says are ongoing abuses, the US has in recent months issues a series of sanctions against actors in Xinjiang, including senior officials and the Xinjiang Production and Construction Corps that operates as a government-within-a government within the resource-rich region.