CHICAGO (AP) _ Leaders of United Airlines' 5,000 striking pilots reaffirmed Friday they will not act on a proposed contract until flight attendants have a back-to- work agreement, even as the airline warned delay could scrap the pilots' pact.

United spokesman Joe Hopkins said Friday that under terms of the tentative agreement with pilots, the proposal will be nullified if it is not accepted by Saturday by the pilots' master executive council.

''I would interpret that as being by ... midnight tomorrow night,'' Hopkins said.

But spokesman Capt. John LeRoy of the Air Line Pilots Association said attorneys told the union ''extenuating circumstances'' - in this case, the flight attendants' ongoing negotiations - would permit extension of the Saturday deadline.

And LeRoy reiterated the pilots would not return to work without an agreement guaranteeing there will be no penalties for flight attendants who honored pilot picket lines.

He said union attorneys told the pilots they could seek ''legal recourse'' if United attempted to withdraw the agreement.

''The company can say that (the agreement would be nullified after Saturday),'' said LeRoy. ''As to whether we agree with that depends on the circumstances hanging at that time.''

Ralph Colliander, a mediator with the National Mediation Board, declined to comment Friday on contents of the board's proposed settlement, which was tentatively accepted by negotiators for both parties early Wednesday.

The pilots walked off their jobs May 17 in the strike that reached its 29th day Friday. United since has been operating only about 14 percent of its pre- strike service of 1,550 daily flights.

The 27-member master executive council of the pilots' union continued for the second day Friday reviewing the language of the tentative agreement. The council also met Wednesday but did not have a copy of the pact.

In addition to the pilots, representatives of the flight attendants union and the airline also resumed meetings Friday.

''Talks are proceeding. That's all we know,'' said union spokesman Mark Bigelow.

United spokesman Chuck Novak said the meetings began at 10 a.m. ''and they're continuing.''

Discussions between the company and the 13-member master executive council of the Association of Flight Attendants focused on the same back-to-work issues that triggered a breakdown in talks between the company and the pilots after economic issues had been settled May 24, said AFA spokeswoman M.J. Brenne.

The main economic issue in the pilots' strike had been a two-tier wage plan that would put new pilots on a lower scale than those already flying for United.

Meanwhile, attorneys for the union and United were told to appear Monday before U.S. District Judge Nicholas Bua for a hearing on a lawsuit filed by the union charging the air carrier with engaging in unfair labor practices.

Bua postponed the hearing at the request of union attorney Michael Abram. It originally had been scheduled Thursday.

LeRoy said the issues contained in the lawsuit before Bua are the fate of 570 pilots newly trained by United who honored picket lines, and the seniority of strikebreakers.

United has said it does not consider its newly trained pilots its employees. It also said earlier that pilots who worked during the walkout would be given preference in bidding on assignments.