US regulators ordered Boeing to make urgent improvements to the best-selling jet involved in a deadly Ethiopia plane crash - but ruled out grounding the fleet as investigators worked to piece together the aircraft's final moments.

The Ethiopian Airlines Boeing 737 MAX 8 crashed minutes into a flight to Nairobi on Sunday, killing all 157 people on board and prompting airlines across the world to begin withdrawing the model from schedules.

The US Federal Aviation Administration (FAA) said it was working with local authorities and the National Transportation Safety Board and may soon share safety information concerning the aircraft.

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

The FAA said it was ordering Boeing to make improvements to anti-stalling software and the manoeuvring system, giving the company until the end of April to make the updates.A team of US aviation experts from the FAA was at the site on Tuesday.

Tamrat Abera, a witness who saw the plane go down, told The Associated Press smoke was coming out of the rear and the aircraftrotated twicebefore hitting the ground.

Boeing responds

Boeing on Monday confirmed it will deploy a software upgrade to the 737 MAX 8, a few hours after the FAA directive to do so came down.

The company said in the aftermath of October's Lion Air Flight crash that it had, for several months, "been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer". The software upgrade "will be deployed across the 737 MAX fleet in the coming weeks," it said in a statement.

Boeing did not reference Sunday's crash in connection to the software upgrade.

"We are confident in the safety of the 737 MAX and in the work of the men and women who design and build it," Boeing Chief Executive Officer Dennis Muilenburg told employees in an email seen by Reuters news agency on Monday.

"Since its certification and entry into service, the MAX family has completed hundreds of thousands of flights safely.

"Speculating about the cause of the accident or discussing it without all the necessary facts is not appropriate and could compromise the integrity of the investigation," Muilenburg told his staff. (Aljazeera)