Some ground workers at Northwest Airlines rejected a tentative agreement and authorized a strike, the union said on Tuesday. The rejection is likely to send Northwest back to the bargaining table with a union it thought it had secured concessions from.

Workers represented by the International Association of Machinists and Aerospace Workers issued a split decision. About 7,600 clerical, office, fleet and passenger service workers approved their tentative agreement. But 5,600 equipment service and stock clerks voted it down.

"The anger and frustration among all employee groups at Northwest Airlines was evident throughout the voting process," said District 143 President Bobby De Pace. "In the end, Northwest management failed to convince a majority of the workforce that the proposal was appropriate or that they could be trusted to honor their commitments."

An airline spokesman did not have an immediate response to the vote.

Voting on the tentative agreement wrapped up on Monday. The union's 14,000 members cast paper ballots that had to be counted by hand.

Northwest, which filed for bankruptcy protection in September, has been pushing for permanent pay cuts and work-rule changes for its workers. The IAM was the first of the large unions to agree.

In January, the IAM announced that members would vote on accepting an 11.5 percent pay cut as well as rules that would allow Northwest to use more part-time workers. The agreement scaled back Northwest's outsourcing proposals, and shifted the airline's pension plan for those workers to the IAM National Pension Plan.

Union leaders said the IAM saved 649 of 733 jobs that Northwest had wanted to outsource, and kept 82 percent of its jobs overall.

Flight attendants and pilots made deals on March 1 and 3. Those have have yet to be ratified by members. Both have authorized strikes if the tentative agreements are not ratified.