Boeing said it was ready to look for another location to build its newest jet if assembly workers and local politicians do not ratify plans to build it in the Seattle area, its traditional manufacturing base.

The US planemaker issued the warning just hours after senior members of the International Association of Machinists union voiced strong opposition to a proposed labour contract that is due to go to a membership vote next Wednesday.

"Vote no and prepare for a strike in 2016," said one of hundreds of comments, most of them negative, on the union's Facebook page.

The discord appeared to threaten tentative deals reached this week that would lock in labour costs and secure Boeing substantial tax breaks in exchange for building the 777X in the Seattle area, where it builds the current 777.

Political leaders said union opposition to the contract only heightened the need for legislative action on tax breaks and other incentives for Boeing.

"This makes it even more urgent," said Alex Pietsch, chief of the Washington governor's office of aerospace. "We need to demonstrate the state is firmly standing behind the company and the machinists."

The state senate appropriations committee was due to hear testimony on the measures, following a similar hearing in the house on Thursday that was broadly in favour of granting about $8 billion in tax breaks to aerospace and funding a $10 billion transportation infrastructure package.

Governor Jay Inslee proposed the package this week calling a special session of the legislature that began on Thursday.-Reuters