FILE PHOTO: An American Airlines Boeing 737 MAX 8 flight from Los Angeles lands at Reagan National Airport shortly after an announcement was made by the FAA that the planes were being grounded by the United States in Washington, U.S. March 13, 2019. REUTERS/Joshua Roberts

WASHINGTON (Reuters) - American Airlines Group Inc AAL.O will extend cancellations of Boeing Co BA.N 737 MAX flights through April 6, it said on Thursday, a day after the U.S. Federal Aviation Administration said it would not approve the plane's return to service before January.

American, the largest U.S. airline, had previously canceled about 140 flights a day through March 4 and now expects to resume 737 MAX passenger flights on April 7.

Once the FAA gives 737 MAX approval, American will need at least 30 days to prepare the jets and its pilots for commercial flights, airline and union officials have said.

Boeing 737 MAXs have been grounded worldwide since March following an Ethiopian Airlines Co crash, the second of two fatal crashes in five months that killed 346 people. Southwest Airlines Co LUV.N Chief Executive Gary Kelly said earlier Thursday it was "likely" the carrier would need to extend its cancellations beyond early March.