FILE PHOTO: United Airlines planes, including a Boeing 737 MAX 9 model, are pictured at George Bush Intercontinental Airport in Houston, Texas, U.S., March 18, 2019. REUTERS/Loren Elliott

CHICAGO (Reuters) - United Airlines Holdings UAL.O said on Friday it is extending the cancellation of Boeing 737 MAX flights by another month until Dec. 19 as the jets remain grounded worldwide following two fatal crashes in Indonesia and Ethiopia in the past year.

Boeing Co BA.N has said it hopes to receive regulatory approval for updated flight control software at the center of both crashes in October, but it could take a month or two for airlines to train pilots on the new software and prepare the jets for commercial flight after sitting idle for months.

As the grounding that began in mid-March has dragged on, airlines that added the fuel-efficient, single-aisle MAX to their fleets in the past two years have had to cancel thousands of monthly flights, eating into their profits.

Chicago-based United had previously pulled its 14 737 MAX jets from its flight schedule through early November. Without the MAX, the carrier said it expects to cancel about 2,800 flights in November and 1,700 through Dec. 19.

Among other U.S. airlines that operate the MAX, Southwest Airlines Co LUV.N has canceled flights into early January due to the grounding, while American Airlines Group AAL.O has canceled through early November, saying it believes it can fly the jets by the holidays.

While the three airlines’ MAX jets accounted for only a fraction of their entire fleet at the time of the grounding, the impact on their schedules has intensified as deliveries of dozens of new orders remain frozen until the planes are approved to fly again.