DENVER -- United Airlines plans to lay off about 600 pilots by the end of the month as it cuts its work force and flight schedule in the aftermath of the Sept. 11 terrorist attacks, a union spokesman said.

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Chicago-based United, the nation's No. 2 carrier, announced Sept. 19 it would reduce its work force by 20 percent, or 20,000 jobs, and its flights by 31 percent after airborne terrorist attacks involving two of its planes dealt a crippling blow to air travel.

Capt. Herb Hunter of the United Air Line Pilots Association said Wednesday the airline plans to lay off about 20 pilot trainees by Sunday, 100 probationary pilots on Oct. 15 and 475 probationary pilots on Oct. 31.

All have less than two years' experience and all could be recalled, he said.

Hunter said United told the union it plans to lay off as many as 250 pilots a month starting in January, but the union was discussing early retirement packages, unpaid leaves with benefits and other alternatives.

About 1,200 United pilots are in the military reserves, and Hunter said as many as 700 could be called to active duty, reducing the need for layoffs.

"We're not sure in our own minds that United will need to do (layoffs) in January," Hunter said. "We want to work with the company to disadvantage as few people as possible and move on."

Hunter said United also plans to offer retirement or transfers to 200 second officers age 60 or older as the airline removes Boeing 727 and 737-291 aircraft from its fleet.

About 1,800 United pilots are based in Denver. Hunter did not know how many would lose their jobs.