Boeing has announced plans to upgrade software in its 737 MAX 8 planes "in the coming weeks", as regulators scrutinise two fatal crashes of the new model of aircraft since October.
The move to deploy the software upgrade came a few hours after the US Federal Aviation Administration (FAA) said it would mandate "design changes" in the aircraft by April.
In making the statement, Boeing did not reference Sunday's Ethiopian Airlines crash, but it did express the company's condolences to the relatives of the 157 people who died.
The United States has said the Boeing 737 MAX 8 is airworthy, as regulators scrutinise the Ethiopian Airlines crash and the crash of a Lion Air jet in Indonesia on October 29, which killed all 189 people on board.
In a notice, the FAA said Boeing was working to complete "flight control system enhancements, which provide reduced reliance on procedures associated with required pilot memory items".
Singapore Airlines has become the latest carrier to suspend the 737 MAX 8.
The company says its SilkAir fleet of six 737 MAX 8 planes will be temporarily withdrawn from service, adding in a Facebook statement that "the safety of our customers and crew is our highest priority".
China and Indonesia also grounded their fleets of 737 MAX 8 aircraft earlier on Monday, citing safety concerns, contributing to a drop in Boeing shares that wiped billions of dollars off the market value of the world's biggest plane maker.
"This is welcome information on the enhancements to address shortfalls that our pilots noted after Lion Air, which is a silver lining," said Dennis Tajer, a spokesman for the American Airlines pilot union and a 737 pilot.
Boeing says it has been working on updates for months
The FAA also said Boeing "plans to update training requirements and flight crew manuals to go with the design change" to an automated protection system called the Manoeuvring Characteristics Augmentation System or MCAS.
The company said in the aftermath of the Lion Air crash that it had "been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer".
The changes also include MCAS activation and angle of attack signal enhancements, which was a function that Lion Air pilots fought in an accident that brought down the same model of jet.
Reuters and other media outlets have reported that Boeing had for months planned design changes after the Lion Air crash in Indonesia but the FAA notice was the first public confirmation.
An Ethiopian Airlines MAX 8 bound for Nairobi crashed minutes after take-off on Sunday, killing all 157 onboard and raising questions about the safety of the new variant of the industry workhorse, one of which also crashed and killed 189 people in Indonesia in October.
The discovery of black box recorders means the cause of the crash may be quickly understood, as long as recordings are not damaged, although it typically takes a year for a detailed investigation to be completed.
The FAA said in the notice made public that external reports are drawing similarities between the crashes in Ethiopia and Indonesia.
"However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions," according to the Continued Airworthiness Notification to the International Community for Boeing 737 MAX 8 operators.
"I want people to be assured that we take these incidents, these accidents, very seriously," said US Transportation Secretary Elaine Chao said.
In an email to employees that was seen by Reuters, Boeing chief executive Dennis Muilenburg said he was confident in the safety of the 737 MAX.
Democratic US senator Dianne Feinstein urged the FAA to ground Boeing's 737 MAX 8 fleet.
"Until the cause of the crash is known and it's clear that similar risks aren't present in the domestic fleet, I believe all Boeing 737 Max 8 series aircraft operating in the United States should be temporarily grounded," Ms Feinstein wrote.
Boeing may face lawsuits from victims' families in the US, where legal compensation payments for people killed in plane crashes could run around $US2 to 3 million per person ($NZ2.2 to 4.4m), depending on the law applied, compared to about $US200,000 in Ethiopia, according to Justin Green, a New York-based aviation lawyer who has represented families in cases against Boeing.
Boeing declined to comment on its insurance cover.
The company's share price briefly had its biggest one-day drop since the 9/11 attacks of 2001, falling as much as 13.5 percent early on Monday on fears that two crashes in such a short time could reveal flaws in the new plane.
On Monday, FAA chief Dan Elwell said the notification basically "informs the international community where we are and (gives) sort of … one answer to the whole community".
Paul Hudson, the president of website Flyers Rights and a member of the FAA aviation rulemaking advisory committee, on Monday said the plane should be grounded.
"The FAA's 'wait and see' attitude risks lives as well as the safety reputation of the US aviation industry," Ms Hudson said in a statement Monday.
The National Transportation Safety Board and the FAA are both at the crash site in Ethiopia, Ms Chao said.
Boeing's safety reputation hits turbulence
Boeing, whose shares closed down 5.3 percent at $US400.01 in the heaviest trading since July 2013, did not immediately comment on the FAA notification.
The 737 series, which has flown for more than 50 years, is the world's best-selling modern passenger aircraft and viewed as one of the industry's most reliable.
China ordered its airlines to ground the jet, a move followed by Indonesia and Ethiopia and individual carriers such as South Africa's Comair and Morocco's Royal Air Maroc.
Virgin Australia has ordered 30 of the same jets, but none are currently in circulation.
Other airlines, from North America to the Middle East, kept flying the 737 MAX 8 on Monday after Boeing said it was safe.
On Tuesday evening, Australia's Civil Aviation Safety Authority (CASA) said it was suspending operations of the plane in Australia.
Boeing's 737 MAX is the newest version of a jet that has been a fixture of passenger travel for decades and the cash cow of the world's largest aircraft maker, competing against Airbus's A320neo family of single-aisle jetliners.
The MAX has a bigger and more efficient engine compared to earlier 737 models.
Boeing rolled out the fuel-efficient MAX 8 in 2017 as an update to the already redesigned 50-year-old 737, and had delivered 350 MAX jets out of the total order tally of 5011 aircraft by the end of January.