Lockheed Martin said it had reached a tentative agreement Saturday night with the machinists union to end a nine-week strike at its fighter jet plant in Fort Worth and two other sites.


The company and the union said that they had agreed not to disclose the terms before the union briefed its members, who will vote on the deal in the next few days. About 3,600 workers went on strike in April over proposed changes in health benefits and a Lockheed plan to stop offering a traditional pension to newly hired workers.


Tensions escalated as the company hired 450 temporary workers, and both sides met with federal mediators from Wednesday through Saturday. Greg Karol, Lockheed’s vice president for labor relations, said in a statement that those discussions prompted the company to revise its offer.


Top Lockheed executives had said repeatedly that the company would not budge on the pension issue.


The Fort Worth factory builds the new F-35 strike fighter aircraft as well as an older model, the F-16. Lockheed, the nation’s largest military contractor, had used salaried workers since the strike began to keep building the planes at a slower rate.


Bob Wood, a spokesman for District Lodge 776 of the International Association of Machinists and Aerospace Workers, had criticized the hiring of temporary workers as a wasteful “dog and pony show.”


But the union had lost leverage in recent days. More than 570 of the strikers had returned to work, and the National Labor Relations Board rejected several union complaints against the company. Pentagon officials had said they would remain neutral on the strike.