Louisville, Kentucky - The US auto workers strike expanded Wednesday with 8,700 more employees walking off their jobs, said the United Auto Workers (UAW) union, as a deal with major carmakers remained elusive.

The staff at Ford's Kentucky Truck Plant in Louisville join some 25,000 other UAW members on strike against the "Big Three" companies – General Motors, Ford, and Stellantis.

The UAW launched a targeted strike on September 15 on GM, Ford, and Stellantis, when a previous contract expired without a replacement.

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

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But the latest move came "after Ford refused to make further movement in bargaining," the UAW said in a statement.

In a separate statement, Ford called the latest strike decision "grossly irresponsible," saying it had put an "outstanding offer" on the table.

The Kentucky plant produces Ford Super Duty pickup trucks, Ford Expeditions, and Lincoln Navigators, and Ford added that the vehicles produced at the facility generate $25 billion a year in revenue.

Ford warned that the widened strike affects direct employees and work stoppage "will generate painful aftershocks," such as impacting other company operations and suppliers.

While previous expansions of the strike took place at a deadline set in advance by the union, the UAW statement dubbed the latest measure a "surprise move."

This also comes a day before the four-week mark since contracts expired at the three major automakers, UAW said.

"We have waited long enough, but Ford has not gotten the message," said UAW President Shawn Fain.

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"It's time for a fair contract at Ford and the rest of the Big Three," he added in a statement.

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.

The three companies have sent home around 3,500 workers in total as of early October, due to knock-on effects from the strike.