WASHINGTON/BEIJING (REUTERS) - China has ordered the cancellation of more than two dozen scheduled flights from the United States in recent weeks after numerous passengers tested positive for Covid-19 after arriving in China.

China's aviation regulator has mandated the cancellations of a total of eight scheduled US passenger airline flights for Shanghai under its Covid-19 pandemic rules: four by United Airlines and two each from Delta Air Lines and American Airlines.

Delta said it cancelled Detroit to Shanghai flights last Friday and this Friday (Jan 7 and 14) due to the Chinese rule requiring "all affected carriers", whose passengers test positive for Covid-19, "to cancel inbound service on certain China flights".

The Civil Aviation Administration of China (CAAC) has also cancelled at least 22 other US-bound flights operated by Chinese carriers since December after positive Covid-19 tests were returned, including eight by China Southern Airlines Co.

The US is facing a surge in infections caused by the highly contagious Omicron variant and on Monday had 132,646 people hospitalised with Covid-19, surpassing the record of 132,051 set in January 2021. The seven-day average for new cases has doubled in the last 10 days to 704,000.

United said it had been forced to cancel flights from San Francisco to Shanghai scheduled for Jan 15, 19, 22 and 26. The Chicago-based carrier flies from San Francisco to Shanghai four times weekly.

The US Transportation Department did not immediately comment late on Monday.

Since the Covid-19 pandemic, China and the US have sparred over air services.

In August, the US Transportation Department limited four flights from Chinese carriers to 40 per cent passenger capacity for four weeks after Beijing imposed identical limits on four United Airlines flights.

China told United in August it was imposing curbs on some flights after it alleged five passengers who travelled from San Francisco to Shanghai tested positive for Covid-19 on July 21.

The department said in August that China's policy "places undue culpability on carriers with respect to travellers that test positive for Covid-19 after their arrival in China".

It said carriers "have no means to independently verify positive test results alleged by Chinese authorities".

A longstanding US-China air agreement allows the countries to operate more than 100 weekly flights between the two nations, but only a fraction of those are currently operating.

Former US president Donald Trump in January 2020 barred nearly all non-US citizens who had been in China within the last 14 days from travelling to the US.

President Joe Biden in November lifted the Chinese travel restrictions for fully vaccinated foreign air travellers.