Employees at US industrial automaker Mack Trucks voted down a labor agreement on Sunday and will join some 25,000 other United Auto Workers (UAW) members on strike, the union announced.

"UAW members at Mack Trucks have voted to REJECT a tentative agreement, and will STRIKE at 7 am on Monday," the union posted on X, formerly Twitter.

The post said that it would impact 4,000 employees at Mack, a maker of industrial vehicles which is headquartered in North Carolina, but has manufacturing plants in Pennsylvania and Maryland.

"I'm inspired to see UAW members at Mack holding out for a better deal, and ready to stand up and walk off the job to win it," said UAW president Shawn Fain.

The Mack workers will be joining striking UAW members at the so-called "Big Three" -- GM, Ford and Stellantis -- who launched a targeted action on September 15 after failing to reach a new deal.

The targeted strike called for stoppages at a handful of plants, while leaving most of the union's 146,000 US hourly auto workers on the job.

After two weeks of expanding the strike to other plants, Fain on Friday said that the UAW would hold off for now, citing last-minute progress in talks.

With the dual earlier expansions of the strike the prior two Fridays, the total number of workers on strike at the Big Three stands at about 25,000.

The industrial action is the first-ever joint strike at the three major automakers in a push for higher salaries and other improvements, especially related to the transition to manufacturing electric vehicles.

In a letter to Mack posted on X, the UAW cited wage increases, cost of living adjustments and several benefits as outstanding issues in their contract negotiations.