The United States has said there is "no basis" to ground Boeing 737 MAX airliners, after a second deadly crash involving the model in less than five months prompted governments worldwide to ban the plane.
Despite the aviation giant's assurances that the plane is safe and reliable, the European Union, Britain and India joined China and other countries grounding the plane or banning it from their airspace as they await the results of the investigation into the crash.
But the US has so far refused to take similar action against the American aerospace giant's best-selling workhorse aircraft.
"Thus far, our review shows no systemic performance issues and provides no basis to order grounding the aircraft," Federal Aviation Administration (FAA) chief Daniel Elwell said in a statement on Tuesday.
A new Ethiopian Airlines 737 MAX 8 went down minutes into a flight from Addis Ababa to Nairobi on Sunday, killing all 157 people on board.
That followed the October crash of a new Lion Air jet of the same model in Indonesia, which killed 189 people shortly after takeoff from Jakarta.
The widening action against the aircraft has put pressure on Boeing -- the world's biggest plane manufacturer -- to prove the 737 MAX is safe, and the company has said it is rolling out flight software updates by April that could address issues with a faulty sensor.
The full extent of the impact of the bans on international travel routes was unclear. There are about 350 MAX 8s currently in service around the world.
Air Canada, for example, announced it was canceling flights to London following Britain's decision to ban the aircraft.
The EU aviation safety agency also closed European airspace to MAX planes.
"At this early stage of the related investigation, it cannot be excluded that similar causes may have contributed to both events," it said, referring to the Ethiopian and Lion Air crashes