The FAA expects to receive the final package for Boeing's software improvements on the 737 Max aircraft in "coming weeks," the agency said Monday – but will not approve the fixes for installation until a "rigorous" review.

The statement comes as the certification process for Boeing's Max jetliners is under intense scrutiny after two accidents that left 346 people dead.

Boeing last week proposed software upgrades to the Max aircraft that focused on a flight-control system known as MCAS that is designed to keep the plane's nose from pitching up, which can result in a deadly stall.

The system is at issue in the March 10 crash of Ethiopian Airlines Flight 302 and the crash of Lion Air Flight 610 that slammed into the Java Sea on Oct. 29.

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"Time is needed for additional work by Boeing as a result of the ongoing review of the 737 Max Flight Control System to ensure that Boeing has identified and appropriately addressed all pertinent issues," the FAA statement said. "Upon receipt, the FAA will subject Boeing's completed submission to a rigorous safety review. The FAA will not approve the software for installation until the agency is satisfied with the submission."

Boeing, which previewed the flight-control changes at its Renton, Washington, facilities, has also said it will mandate more pilot training.

The FAA statement also comes as an initial report on the Ethiopian crash was expected within days. The two tragedies showed haunting parallels: Both flights experienced drastic speed fluctuations during ascent, and each pilot made a futile attempt to return to the airport a few minutes after takeoff.

Ethiopian Airlines CEO Tewolde GebreMariam has said the stall-prevention system, a new feature on the MAX models that debuted in 2017, was activated on his company’s doomed flight.

The Max aircraft have been grounded around the world since mid-March and the FAA has been under fire for acting too slowly in parking the planes.

Contributing: Chris Woodyard and Jorge Ortiz, USA TODAY