An American Airlines Boeing 737 Max 8, on a flight from Miami to New York City, comes in for landing at LaGuardia Airport in New York, March 12, 2019.

The prolonged grounding of Boeing's 737 Max jets forced American Airlines on Tuesday to cut its revenue guidance for the first quarter.

The company reduced its total expected revenue per seat mile, a key industry measure of performance, to flat to up to 1% from its previous guidance of flat to up to 2%.

Its fleet of about 963 aircraft includes 24 Max jets, which have been grounded following the March 10 crash of Ethiopian Airlines Flight 302 over Addis Ababa. American canceled 1,200 flights during the first quarter as a result of the grounding, and will extend flight cancellations through June 5, over a month longer than previously announced.

The airline said it now expects first-quarter consolidated cost per available seat mile, excluding fuel and special items, to be up about 3% from a year earlier, a decline from the company's previous guidance as a result of lower anticipated salaries and benefits expense.

American also expects to pay on average $2.02 to $2.07 per gallon of mainline jet fuel in the first quarter, higher than previously guidance of $1.97 to $2.02.

Pretax margin estimates are now 2% to 4% percent, lower than previously estimated due to an increase in fuel prices, the airline said.

During the first quarter, American repurchased 16.7 million shares at a total cost of $600 million and said that the fully diluted weighted average share count for the first quarter was roughly $454 million.

Regarding the 737 groundings and resulting flight cancellations, American said it cannot forecast the resulting financial implications in future periods since it doesn't know how long the aircraft will be grounded or when it will back in service.

On Monday, Wall Street analysts downgraded shares of Boeing and Southwest Airlines. The move followed Boeing's announcement late Friday that it will cut production of the 737 Max jet by 20 percent as it tries to find a software fix to get it back in the air.

The groundings will likely have less impact on American and United than on Southwest, which has 34 Max jets out of its fleet of about 750 aircraft.

The 737 Max jet's automated flight control system was implicated in the October crash of Lion Air Flight 610 and the March crash of Ethiopian Airlines Flight 302 that killed a total of 346 people.