Mainland China reported 39 new coronavirus cases as of Sunday, up from 30 a day earlier, and the number of asymptomatic cases also surged, as Beijing continued to struggle to extinguish the outbreak despite drastic containment efforts.

The National Health Commission said in a statement on Monday that 78 new asymptomatic cases had been identified as of the end of the day on 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 in recent weeks after draconian containment measures succeeded in slashing the overall infection rate.

Hubei province, the original epicenter of the outbreak, accounted for almost half of the new asymptomatic cases, the provincial health authority said. 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 the Hubei provincial capital of 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 China's wider effort to get the economy back on track even as it tries to prevent a second wave of infections.

Wuhan officials revoked the "epidemic-free" status of 45 residential compounds due to the emergence of asymptomatic cases and for other unspecified reasons, according to a report on Monday by the official Xinhua news agency.

"Epidemic-free" status allows people living in a compound in Wuhan to leave their homes for two hours at a time.

Mainland China has reported a total of 81,708 cases, with 3,331 deaths.

Cases cross Russian border

China has closed off its borders to foreigners as the virus spreads globally, though most imported cases have involved Chinese nationals returning from overseas.

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 neighboring Russia, the provincial government said. All were Chinese citizens who had flown from Moscow to Vladivostok, including 12 who were on the same April 2 flight and traveled to China by road.

Another possible source of infection is the roughly 1.6 million Chinese citizens who study overseas, many of whom have struggled to return home since Beijing cut international flights into the country.

Preparations are underway to arrange charter flights to bring home Chinese students studying in the United States, starting with the youngest, China's embassy to the United States said.

One new locally transmitted infection was reported in the latest data, in the southern province of Guangdong, down from five a day earlier in the same province.

The new locally transmitted case in the city of Shenzhen was a person who had traveled from Hubei province, Guangdong provincial authorities said.

The Guangdong health commission raised the risk level for a total of four districts in the cities of Guangzhou, Shenzhen and Jieyang from low to medium late on Sunday.