United Airlines said Friday it pushed back its expected date for Boeing's 737 MAX aircraft to return to service, following similar announcements by rivals Southwest and American Airlines.

The US air carrier now says it expects flights to resume on March 4, 2020, two months later than a previous estimate, implying the cancellation of thousands of scheduled flights.

The top-selling Boeing jets have been grounded since March of this year following the second of two crashes which left 346 people dead.

Boeing has faced stumbling blocks in its efforts to win global regulators' approval for proposed fixes to the aircraft and get the planes flying again.

In statement, United said it was working to minimize disruptions for the flying public, rescheduling flights or making other offers.

"We have cooperated fully with the FAA's independent review of the MAX aircraft, and we won't put our customers and employees on that plane until regulators make their own independent assessment that it is safe to do so," it said.

United had previously targeted a return to service by January 2020
United had previously targeted a return to service by January 2020 GETTY IMAGES NORTH AMERICA / MARIO TAMA

Boeing says it hopes to get regulatory approval for a return to service before the end of this year but has delayed its estimate for the resumption of commercial flights until January, to allow for changes to pilot training.

Southwest, the world's biggest buyer of 737 MAX jets, with 34 of them in its fleet at the time of the grounding in March, said earlier this month it now expects a return to service on March 6, 2020.

American Airlines, which purchased 24 of the jets, said it expected its 737 MAX jets to fly again on March 5.

The Southwest pilots union on Wednesday denounced what it said was Boeing's pressure on regulators to speed up authorization for the 737 MAX's return to service.

"Boeing is increasingly publicizing that they may have to shut down their production line due to running out of room to store completed MAX aircraft," the statement said.

"There is some concern that this is simply another tactic" to speed up the return to service and push some costs back on to operators, it added.

"Boeing will never, and should not ever, be given the benefit of the doubt again. The combination of arrogance, ignorance, and greed should and will haunt Boeing for eternity."