CHICAGO (AP) _ Striking United Airlines pilots may lose their jobs forever unless they drop their demand for back-to-work agreements for workers outside their union's jurisdiction, company officials say.

On Wednesday, United rejected a call by the National Mediation Board to resume talks today, saying it would not negotiate until the union changed its position on a back-to-work agreement covering people who have refused to cross picket lines during the 21-day strike.

''We quite frankly don't think it's any of their business what we do with management employees, what we do with flight attendants'' or with a pool of 500 new pilots trained by United, said David Pringle, the airline's senior personnel official. The company has said it does not consider those 500 pilots to be United employees.

The mediators said they were still willing to hold a meeting today at their Washington, D.C., headquarters with the Air Line Pilots Association even without United's participation. But after the company declined, the union said it would not attend alone.

Pringle said the union's position made the possiblity of an extended strike more likely.

''If the strike goes on for a month, I think it's likely to go on for 10 months,'' he said.

And if the union's demands are not withdrawn, it would be ''difficult, if not impossible'' to guarantee the striking pilots their old jobs, said John R. Zeeman, senior vice president for marketing for the airline.

Bill Brashear, chief negotiator for the union, also said that United Chairman Richard Ferris may feel that ''his credibility is on the line,'' because ''he has made a commitment that these 500 new pilots would not work for the airline.''

In addition to the 500 pilots, the union wants assurance there will be no reprisals against 10,000 flight attendants, most of whom refused to cross picket lines. The company says the union is not authorized to bargain for these employees.

In addition, the union opposes the company's establishing a job-bidding list giving preference to pilots who crossed picket lines.

Attempts to revive talks came almost two weeks after negotiations broke down May 25 over a back-to-work agreement, even though contractual issues had been resolved.

Tentative settlement had been reached on the main economic issue - a two- tier salary wage that would put newly hired pilots at substantially lower wage scale than veterans already flying for the airline.

United has been operating 209 flights to 41 airports, about 14 percent of of its pre-strike daily service.