The Federal Aviation Administration has told Boeing it will order the company by next month to institute safety-related software enhancements to its 737 Max 8 planes, Boeing said late Monday. Boeing's statement didn't mention the crash Sunday in Ethiopia of an Ethiopian Airlines Max 8 that killed all 157 people on board, including 8 Americans.

But it did say the changes have been in the works since a Lion Air Max 8 crashed into the sea off Indonesia in October, taking the lives of the 189 people on that aircraft.

Boeing said the improvements would be deployed across the entire Max 8 fleet in coming weeks, adding that they're "designed to make an already safe aircraft even safer."

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The changes will include updates of an automated anti-stall system suspected of contributing to the Lion Air crash and of training requirements and flight crew manuals related to the system, known as the Maneuvering Characteristics Augmentation System, or MCAS.

The system automatically points the plane's nose down if sensors indicate the plane could be in danger of losing lift, or stalling. Sensors on the plane operated by Indonesia's Lion Air gave out faulty readings on its last four flights.

On the fatal October flight, pilots apparently struggled in vain to fight against the automated nose-down commands.

The FAA tried to discourage comparisons between the Lion Air crash and Sunday's deadly crash. Both performed erratically shortly after takeoff, then went into nosedives. Weather didn't appear to be a factor in either one.

"External reports are drawing similarities between (the Ethiopian) accident and the Lion Air Flight 610 accident," the FAA said. "However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

Transportation Secretary Elaine Chao said her department, which includes the FAA, was "very concerned" and monitoring developments around Sunday's crash. She said she met with acting FAA Administrator Dan Elwell to discuss the situation "and what are our possible paths forward."

She didn't say whether the agency was considering grounding any planes, but the FAA said Monday it still considered the Max 8s to be airworthy.

Experts from the FAA and National Transportation Safety Board were in Ethiopia Tuesday. They joined an Ethiopian-led investigation that included authorities from neighboring Kenya and elsewhere.

A growing number of countries and airlines have grounded Max 8s in the wake of Sunday's crash.

One witness has told The Associated Press smoke was coming from the plane's rear before it crashed.

A consumer group, FlyersRights.org, urged the FAA to ground the Max 8.

"The FAA's wait-and-see attitude risks lives," said the group's president, Paul Hudson.

U.S. airlines repeated their belief that the plane is safe.

American Airlines, which operates 24 Max 8s, said it bases its judgment on collecting extensive data on its entire fleet, including the Max 8.

"We have full confidence in the aircraft and our crew members," the carrier's vice president of flight service, Jill Surdeck, said in a memo to employees.

Southwest Airlines operates the largest U.S. fleet of Max planes, with 34 Max 8s. Spokesman Brian Parrish said the airline remains confident in their safety. United has about a dozen Max 9s, which are slightly bigger than the Max 8.

Dozens of Max aircraft crisscrossed the skies over the U.S. Monday, and passengers continued to board them and fly without incident. Passengers interviewed at Houston's Hobby Airport were keenly aware of the crash in Ethiopia. They expressed concern but no panic.

The 737 Max is the newest version of the 737, the best-selling airliner ever. Since debuting in 2017, Boeing has delivered more than 350 of them in several versions that vary by size.

Dozens of airlines around the world have embraced the plane for its fuel efficiency and utility for short and medium-haul flights.

Boeing has taken more than 5,000 orders for the various Max versions, and they constitute the largest share of the company's backlog of nearly 5,900 planes. They carry list prices from $100 million to $135 million, although airlines routinely get deep discounts.