China's civil aviation regulator on Thursday ordered further cuts to flights in and out of the country as part of the efforts to contain the increasing risks of COVID-19 cases coming from other countries.

Effective from March 29, all domestic airlines will be allowed to operate only one flight to each country per week, while foreign airlines are instructed to reduce their international routes to China to one per week and only operate one route into the country, according to the Civil Aviation Administration of China (CAAC).

The regulator also instructed all airlines to file flight plan applications in advance.

"To resolutely contain the increasing risks of imported COVID-19 cases, and in accordance with the requirements of the State Council for joint prevention and control of the pandemic, [the regulator] has decided to further reduce the number of international passenger flights," the CAAC said.

Stricter measures will also be taken to curb the spread of the virus on flights arriving or leaving the country and to ensure that passenger load factor is lower than 75 percent, the CAAC said, adding that it might further slash international flights if necessary.

Airlines will be allowed to use their spare capacity to carry cargo in and out of China, the regulator said.

Around 80 percent of international flights were already cancelled prior to the announcement, but Chinese airlines had been asked not to cut their international routes until Thursday's order by the CAAC.

Imported COVID-19 cases topped 541 in China by the end of Wednesday, official data showed. Hubei, the central China province where the virus broke out, reported no new cases on Wednesday and opened its borders.