SEATAC, Wash. — Leaders of Boeing Co.'s Machinists union called Friday for a strike after rejecting the aerospace giant's "best and final" contract offer. They urged union members to reject the offer in a vote set for Wednesday.

The Chicago-based company hoped the proposal, which provides added pay and incentives to workers over three years, would help it avert a labor standoff. The talks come as Boeing tries to keep up with a backlog of plane orders and avoid more penalties caused by production delays of its next-generation passenger jet.

Tom Wroblewski, district president of International Association of Machinists and Aerospace Workers District Lodge 751, said the company's offer fell short in terms of job security, wages and medical coverage, among other areas.

Boeing spokesmen said the company was "extremely disappointed" by the union's response. The current contract expires Wednesday.

Wroblewski said the union was willing to continue bargaining and would welcome the involvement of a federal mediator.

"We are more than willing to talk to the union and discuss the contract with them in more detail — that said, this is truly our final offer," Boeing spokesman Jim Proulx said late Friday.

The union held a preliminary vote to authorize a strike in July. On Wednesday, members are scheduled to cast two ballots: one to accept or reject Boeing's latest offer and another on whether to begin a strike.

The sides have been negotiating since May 9; round-the-clock talks began Aug. 21 at a hotel near a Seattle airport.

In 2005, about 18,400 machinists in the Pacific Northwest and Wichita, Kan., struck for four weeks, forcing the company to halt commercial airplane production. The machinists assemble Boeing's commercial planes and key components.

The proposal, Boeing's third offer, was delivered to the union Thursday. It would have increased pay by 11 percent on average for more than 27,000 union workers in Washington state, Kansas and Oregon, the company said.

The tentative deal also included a $2,500 bonus for workers if the agreement was ratified by Wednesday.