The National Health Commission (NHC) said on Monday that 78 new asymptomatic cases had been identified as of the end of Sunday, compared with 47 the day before.
Imported cases and asymptomatic patients, who show no symptoms but can still pass the virus to others, have become China's chief concern after draconian containment measures succeeded in slashing the overall infection rate.
"The risk of imported cases from our neighbouring countries keeps rising," NHC spokesman Mi Feng told a news conference, adding that China needed to keep alert and prevent a resurgence of the epidemic.
Hubei province, the original epicentre of the outbreak, accounted for almost half the new asymptomatic cases. A total of 705 people with asymptomatic cases were under medical observation around mainland China.
The surge in asymptomatic cases, which China only began reporting last week, poses a worry as Hubei's capital Wuhan prepares to allow people to leave the city on April 8 for the first time since it was locked down in late January.
Hubei began easing travel curbs late last month, part of a wider effort to get the economy back on track.
Wuhan officials revoked the "epidemic-free" status of 45 residential compounds due to the emergence of asymptomatic cases and other unspecified reasons, according to a report on Monday by the official Xinhua news agency.
"Epidemic-free" status allows people living in Wuhan compounds to leave their homes for two hours at a time.
China has reported a total of 81,708 cases, with 3,331 deaths.
CASES CROSS RUSSIAN BORDER
China has closed its borders to foreigners as the virus spreads globally, though most imported cases have involved Chinese nationals returning from overseas.
It began testing all international arrivals for the coronavirus from April 1, a customs department official said.
Of the new cases showing symptoms, 38 were people who had entered China from abroad, compared with 25 a day earlier.
Of those, 20 had arrived in the northeastern province of Heilongjiang from neighbouring Russia. All were Chinese citizens who had flown from Moscow to Vladivostok and travelled to China by road.
Another possible source of infection are the 1.6 million Chinese citizens who study overseas, many of whom have struggled to return home since airlines and governments, including the Chinese government, cut international flights.
Charter flights are being arranged to bring home Chinese students studying in the United States, starting with the youngest.
Writing in the New York Times on Monday, China's ambassador to the United States, Cui Tiankai, noted there had been "unpleasant talk" between the two countries about the virus.
"But this is not the time for finger-pointing. This is a time for solidarity, collaboration and mutual support," Cui wrote.
One new locally transmitted infection was reported in the latest China data, in the southern province of Guangdong, down from five a day earlier in the same province.
Guangdong's health commission raised the risk level for a total of four districts in Guangzhou, Shenzhen and Jieyang from low to medium late on Sunday.
In Beijing, Tongren Hospital doctor Wang Chengshuo noted April and early May brought the annual catkin phenomenon – when waves of white balls of tree fluff float through the air – but said there is no evidence catkins can carry the virus.