US aviation regulators say Boeing needs more time to finish changes in a flight-control system suspected of playing a role in two deadly crashes.

The Federal Aviation Administration said on Monday it anticipates Boeing's final software improvements for 737 Max airliners "in the coming weeks".

Boeing was expected to complete the work last week, but FAA says the company needs more time to make sure it has identified and addressed all issues.

The FAA says once it gets Boeing's completed proposal, it will conduct a rigorous safety review.

The planes have been grounded around the world since mid-March.

Boeing is making changes in an automated system that is designed to prevent the plane's nose from rising, which can lead to a dangerous aerodynamic stall. Max crashes in Indonesia and Ethiopia killed 346 people.