A major strike against Pan American World Airways that could have severely curtailed the airline's operations during the height of the tourist season was averted yesterday when the airline and the Teamsters union reached a tentative agreement on a new contract.

The agreement came some 15 hours after the union, which represents about 5,500 Pan Am reservations agents and other ground personnel, called on its members to cease working after negotiators failed to reach agreement on a contract by a 12:01 a.m. deadline.

Although pickets lines went up outside many Pan Am facilities in the U.S. and its possessions, and ticket offices were closed yesterday, Pan Am flights continued to operate, reporting delays between 30 minutes and several hours.

During the walkout, Pan Am reported that most other employes showed up for work and that management personnel filled in for the missing ground staff.

Most of Pan Am's flights are scheduled for evening hours and were not expected to be affected.

The locals in Honolulu and Miami reportedly were defying the international union's instructions to return to work by continuing to picket last night.

Although details of the agreement were not released pending approval at union membership meetings next week, a union spokesman said the sticking point was job security.