Federal mediators yesterday declared a 30-day cooling-off period that would allow the 7,000 union mechanics at US Airways to strike the airline Sept. 26 if agreement is not reached on a new contract.

The action by the National Mediation Board, which governs labor relations in the airline and railroad industries, ends a year of federal mediation efforts. Once the cooling-off period ends, the only way to stop a strike is through presidential intervention, which appears unlikely.

Robert Roach Jr., International Association of Machinists vice president for transportation, said the union was prepared to return to the bargaining table. But he said: "We've been in negotiations for four years. It's time to bring this to an end."

Roach said the union negotiators have the ability and the authority to negotiate a settlement and "the rest is up to the company."

In a telephone hot-line message to its members, the union said, "If tentative agreement is not reached, the strike will begin at 12:01 a.m. Sept. 26."

US Airways would not comment on the strike threat. But Chairman Stephen M. Wolf and President Rakesh Gangwal issued an internal memo to employees urging them to continue to "function as capable professionals" while the company tries to reach an agreement with the union during the cooling-off period.

The two men praised the employees for what they described as a difficult year during which weather and mechanical failures have forced many flight delays and cancellations. Some airline officials and analysts have suggested the mechanics added to the problem by requiring more inspections before signing off on mechanical repairs.

"We realize this year has been challenging for you, particularly for our frontline employees who have had to deal with difficult operations and considerable customer dissatisfaction," the two men said in their memo. "We deeply appreciate the way you have handled these extra burdens. At the same time, we especially want to take this opportunity to offer our sincere thanks to the vast majority of our IAM members who continue to do a superb job."

US Airways mechanics have been in negotiations since 1995. Last June, the two sides reached tentative agreement on a new three-year contract, but the union membership overwhelmingly rejected the agreement last month.

The decision by the government to "release" the two sides from government control and declare a cooling-off period now puts new pressures on both sides to reach contract agreement. If agreement is not reached it would be unlikely that President Clinton would appoint a special emergency board to settle a strike after refusing to do so when the Air Line Pilots Association struck Northwest Airlines last year, crippling the airline's operations.