Air Canada has more customer complaints about refunds to the U.S. Department of Transportation than any non-U.S. airline.

The dispute between Washington and Beijing over airline service has been building for weeks and is part of broader trade and diplomatic tension between the world's two biggest economies.

In an order posted online, the U.S. Transportation Department noted Delta Air Lines and United Airlines want to resume flights to China in June, even as Chinese carriers have continued U.S. flights during the COVID-19 pandemic.

He said passengers on the United Airlines flight were "scared" and "shocked".

The Transportation Security Administration screened 94,931 people on Wednesday, a drop of 96 per cent from a year ago.

United said Wednesday that starting in April it will reduce passenger-carrying capacity 20 per cent on international routes and 10 per cent in the U.S.

"You should exercise a high degree of caution at all times, monitor local media and follow the instructions of local authorities," the Canadian federal government says.

A spokesperson for the airline said in a statement that the incident occurred on a flight from San Francisco to Atlanta.

The woman reportedly spent an hour stuck in the bathroom before being freed.

United owns 14 Max jets, which have been grounded since March after the second of two accidents that together killed 346 people.