Southwest Airlines and American Airlines on Friday pushed back their timeframe again for resuming flights on the Boeing 737 MAX amid uncertainty over when regulators will clear the plane for service.

Southwest extended its grounding through March 6, 2020, while American extended it until March 5.

The planes have been grounded globally since mid-March following two crashes that killed 346 people. The grounding has dragged on much beyond initial expectations as Boeing has upgraded systems and faced questions from regulators and politicians over the plane.

Domestic-focused Southwest, which had previously set February 8 as a return date for the MAX, said "proactively" removing the aircraft from service reduced the likelihood of last-minute cancellations and disruption.

"We remain confident that, once certified by the FAA, the enhancements will support the safe operation of the MAX."

A few hours later, American announced a similar move after previously grounding the plane through January 15.

American said adjusted the timeframe followed "continuous contact" with the Federal Aviation Administration and Boeing.

Boeing has said it expects to receive regulatory approval to resume flights in the fourth quarter of 2019, but that timeframe has started to look uncertain as the end of the year approaches.

Boeing did not immediately respond to a request for comment Friday.