US regulators have ordered Boeing to make urgent improvements to its popular 737 Max 8 aircraft after the second deadly accident in a few months prompted the grounding of fleets around the world.

Singapore Tuesday banned the use of the high-tech jet in its airspace, becoming the latest in a growing list of jurisdictions worried about the safety of the flagship plane.

On Sunday an Ethiopian Airlines Boeing 737 MAX 8 went down minutes into a flight to Nairobi, killing all 157 people on board. It came after a Lion Air jet of the same model crashed in Indonesia in October, killing 189.

Investigators have recovered the black box flight recorders from the Ethiopian Airlines plane, which went down near Addis Ababa, and was carrying passengers and crew from 35 countries, including some two dozen UN staff.

The US Federal Aviation Administration (FAA) said Monday it was ordering Boeing to improve anti-stalling software and the model’s maneuvering system, giving the company until the end of April to make the updates.

But the body ruled out grounding the fleet for now. It said investigations had “just begun” and so far no data had been provided to “draw any conclusions or take any actions.”

“If we identify an issue that affects safety, the FAA will take immediate and appropriate action,” it said in a statement.

Singapore regulators’ announcement that all types of MAX aircraft were banned from its airspace comes after China ordered domestic airlines to suspend commercial operations of the MAX 8, and Indonesia grounded its entire fleet of the jets for inspections.

Ethiopian Airlines has grounded its remaining MAX 8 jets, while airlines in South Africa, Brazil and Mexico have taken theirs out of service and pilots from Argentina’s Aerolineas Argentinas are refusing to fly them.

The Malaysian government ordered an urgent review of orders for several MAX aircrafts by flag carrier Malaysia Airlines.

Several airlines have said they are not canceling MAX 8 flights, however, while US carriers appear to retain confidence in the manufacturer.

Fastest-selling planes

Boeing has described the MAX series as its fastest-selling family of planes, with more than 5,000 orders placed to date from about 100 customers.

But not since the 1970s — when the McDonnell Douglas DC-10 had successive fatal incidents — has a new model been involved in two deadly accidents in such a short period.

The weekend crash sent Boeing shares nosediving as much as 12 percent on Monday.

The plane involved in Sunday’s crash was less than four months old, with Ethiopian Airlines saying it was delivered on November 15.

It went down near the village of Tulu Fara, some 60 kilometers east of Addis Ababa.

Inhabitants of the remote area looked on from behind a security cordon as inspectors searched the crash site and excavated it with a mechanical digger.

The single-aisle Boeing jet had left a deep, black crater.

Ethiopian Airlines said the pilot was given clearance to turn around after indicating problems shortly before the plane disappeared from radar.

The airline’s chief executive Tewolde GebreMariam said the plane had flown in from Johannesburg early Sunday, spent three hours in Addis and was “dispatched with no remark,” meaning no problems were flagged.

The crash cast a pall over a gathering of the UN Environment Programme as it opened in Nairobi — at least 22 staff from several UN agencies were on board the doomed flight.

Delegates hugged and comforted one another as they arrived at the meeting with the UN flag flying at half-mast.

Other passengers included tourists and business travelers.

Kenya had the highest death toll among the nationalities on the flight with 32, according to Ethiopian Airlines. Canada was next with 18 victims.

There were also passengers from other countries including Ethiopia, Italy, the US, Britain and France.

Among those on board was Italian archaeologist Sebastiano Tusa, 66, his wife Valeria Patrizia Li Vigni was quoted as saying by the Corriere della Sera newspaper.

On Sunday, “the friends I met at mass said I shouldn’t worry because bad news travels fast,” she said.

“In the end it arrived anyway, and it destroyed my life. I felt the disaster coming… He hadn’t even wanted to go.”