BEIJING, March 26 (Xinhua) -- China's civil aviation regulator issued a circular Thursday to further cut international flights as part of efforts to stem the inflow of novel coronavirus disease (COVID-19) cases.

All domestic airlines were allowed to operate only one flight to each country per week, while foreign aviation companies should keep only one air route to China and there should be no more than one flight every week for each of the air route to China, the Civil Aviation Administration of China (CAAC) said in a circular that will be effective from March 29.

The administration also instructed all airlines to file applications of flight plans in advance as requested by the circular.

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

Imported COVID-19 cases topped 541 in China by the end of Wednesday, official data showed.