Last week, the United Auto Workers union began unprecedented simultaneous strikes at one assembly plant each for General Motors, Ford and Stellantis, but analysts expect any expansion of strikes to include plants producing highly lucrative pickup trucks such as Ford's F-150, GM's Chevrolet Silverado and Stellantis' Ram.

About 12700,146 factory workers went on strike in Missouri, Michigan and Ohio, which produce Ford Bronco, Jeep Wrangler, Chevrolet Colorado and other popular models. Union chairman Sean Fine warned that more union members of the 1600,<> who work at the three companies in Detroit would join unless new agreements are reached before <> GMT on Friday.

The crisis is raising fears of a prolonged strike that could disrupt production, affect the supply chain and hurt U.S. economic growth. A Reuters/Ipsos poll on Thursday showed significant support from Americans for striking auto workers.

The three companies have proposed a 20 percent wage increase over four and a half years, while the union is demanding a 40 percent increase. The union is also demanding the abolition of a tiered pay structure that it says has created a huge gap between new and old employees.

S&P said the strikes, which began on Sept. 15, are likely to last for several weeks, potentially cutting U.S. third-quarter gross domestic product by 0.39 percent and causing "disruptions" to global auto supply chains.

The three companies said they were making contingency plans to counter further downtime in the United States.