A panel of international air safety regulators on Friday harshly criticized the U.S. Federal Aviation Administration's (FAA) review of a safety system on Boeing's 737 MAX airliner later tied to two crashes that killed all 346 people aboard.

The Joint Authorities Technical Review (JATR) was commissioned by the FAA in April to look into the agency's oversight and approval of the so-called MCAS anti-stall system.

The report also faulted Boeing for assumptions it made in designing the airplane and found areas where Boeing could improve processes.

"The JATR team found that the MCAS was not evaluated as a complete and integrated function in the certification documents that were submitted to the FAA," the 69-page series of findings and recommendations said.

"The lack of a unified top-down development and evaluation of the system function and its safety analyses, combined with the extensive and fragmented documentation, made it difficult to assess whether compliance was fully demonstrated."

Boeing did not directly address the report's findings but said it "is committed to working with the FAA in reviewing the recommendations and helping to continuously improve the process and approach used to validate and certify airplanes."

Regulators around the world continue to scrutinize proposed software changes and training revisions from Boeing aimed at returning the Boeing 737 MAX to service.