-- Days before a contract between Boeing Co. and its machinists union is due to expire, labor leaders said the two sides remained far apart and warned of a possible strike.

Machinists Lodge 751 countered the aerospace company's latest contract offer with its own proposal on Monday but would not provide details.

The three-year contract with Boeing's biggest union, which represents 18,400 mostly hourly workers who assemble passenger and cargo jets and build components for those aircraft in the Seattle area, Wichita, Kan., and Gresham, Ore., expires Friday. Under a negotiating timetable cited by both sides, Boeing's final offer is due Tuesday.

The latest Boeing proposal, submitted late Sunday, would boost cash bonuses by at least $1,500, but union leaders said it ignores a more important issue -- pensions.

Union spokeswoman Connie Kelliher said the two sides remained "far apart." She dismissed the company's three-year offer as a "minimal" advance.

"The important thing is that we're still negotiating. We're still discussing the issues," Boeing spokesman Charles Bickers said.

Workers represented in the talks now receive an average of $59,000 a year. No general pay increase has been offered, but Boeing estimates that cost-of-living provisions would boost base wages about 1 percent in each year of the contract.