US aviation giant Boeing has improved the conditions in its contract offer to thousands of striking workers, with a 38 percent wage increase over four years and a ratification bonus, their union said Thursday.

The International Association of Machinists and Aerospace Workers District 751, which represents more than 33,000 workers who went on strike on September 13 in the Seattle area, have endorsed the contract, and scheduled a vote on Monday.

"Your union is endorsing and recommending the latest IAM/Boeing contract proposal. It is time for our members to lock in these gains and confidently declare victory," the IAM chapter said.

This was the fourth offer made by Boeing since early September, but the third on which members have been asked to vote.

Members overwhelmingly rejected an offer of a 25 percent raise over four years on September 12. A second offer, which promised a 35 percent pay rise, was rejected by nearly two-thirds of members last week.

The union has consistently asked for a 40 percent salary increase.

"We believe asking members to stay on strike longer wouldn't be right as we have achieved so much success," the union said.

Beyond the 38 percent wage increase, members will be entitled to a $12,000 ratification bonus, up from $7,000 in the previous offer, Boeing said.

"We encourage all of our employees to learn more about the improved offer and vote on Monday, November 4," Boeing said in a statement.