An American Airlines jet takes off from Washington National Airport in Washington, U.S., August 9, 2017. REUTERS/Joshua Roberts

(Reuters) – American Airlines Co Group Inc said on Tuesday its first-quarter revenue per available seat mile would be below its previous forecast due to the groundings of its 24 Boeing 737 MAX 8 planes and the U.S. government shutdown.

The airline said it now expects here the closely followed measure of airline performance to be flat to up 1 percent compared with the prior forecast of flat to 2 percent growth.

American said it had canceled about 1,200 flights in the first quarter and was unable to forecast the financial costs of disruption for future periods.

The No.1 U.S. airline said it will extend cancellations of 90