Workers who make military helicopters for the giant US aircraft firm Boeing have voted to reject a pay offer and are preparing to strike.

The action could become more widespread if 26,000 machinists in Boeing's commercial jetliner division also vote to strike in a ballot on pay on Thursday.

Leaders of the 26,000 machinists in Boeing's biggest union, the International Association of Machinists and Aerospace workers (IAM), have urged their members to reject the pay deal.

Boeing officials say deliveries of Chinook helicopters to the US military could be delayed if 1,500 workers walk out of its Pennsylvania factory.

Special powers

However, Pentagon guidelines would allow the company to use other workers to cover strikers' jobs.

"We're supplying Chinooks to people in Afghanistan and things like that," said Boeing spokeswoman Madelyn Bush.

"The government requires that we keep doing that."

Both disputes centre on pay deals which the unions say are too low, coupled with changes to payments for health insurance and other benefits.

Like the rest of the aviation industry, Boeing has been hard hit by falling demand for commercial aircraft since the 11 September suicide hijacking attacks as airlines have mothballed planes and cancelled orders.

Hard times

Boeing's Commercial Airplanes division has said orders for 504 aircraft have been delayed by customers since the attacks.

The IAM said the latest contracts offered little job security and poor benefits.

Leaders of the United Aerospace Workers (UAW) union at the Ridley plant near Philadelphia in Pennsylvania have rejected their pay deal on similar grounds.

"I consider that (offer) an insult and I think most of our members consider that an insult," said John DeFrancisco, President of UAW Local 1069 in Pennsylvania.

Boeing's spokeswoman said it was "our best and final offer".