Car workers in the US have expanded their strike again, just hours after General Motors (GM) warned investors that costs from the industrial action remained unpredictable.

The US car giant said the industrial action was expected to cost the company roughly $200m (£164m) each week.

Executives declined to predict when the row over pay and benefits might end.

The United Auto Workers (UAW) union began strike action against GM, Ford and Stellantis in September.

More than 45,000 workers are now participating in the walkout - the first in the union's history to target all three firms at once.

The UAW's latest announcement expanded the stoppage to a GM assembly plant in Texas where about 5,000 union members work. The site makes popular vehicles such as the Chevy Tahoe SUV.

GM said it was disappointed by the escalation, calling the strike "unnecessary and irresponsible".

"It is harming our team members who are sacrificing their livelihoods and having negative ripple effects on our dealers, suppliers and the communities that rely on us," the company said.

"It is time for us to finish this process."

GM executives had earlier told investors that the strike, which the union has been expanding steadily since its start, had cost it about $800m to date.

They said they expected the action to result in about $200m in lost production each week moving forward. GM is withdrawing profit forecasts for the year because it cannot predict how quickly the stand-off will conclude.

The carmaker said it had offered a "record" labour contract in an effort to help resolve the dispute, which is now in its sixth week.

The deal includes a 23% rise in pay, a reinstatement of pay adjustments tied to inflation and other benefits, according to details released by the company last week.

GM said the proposal would leave the average worker with a salary of about $84,000 by the end of the four-year contract.

GM boss Mary Barra, who has been attacked by the union for receiving a pay package of more than $28m last year, said she knew that investors were "concerned about the impact of higher labour costs".

"Let me address this head on. It's been clear coming out of Covid that wages and benefits across the US economy would need to increase because of inflation and other factors," she said.

Ms Barra said the current GM proposal "rewards our team members but does not put our company and their jobs at risk".

• Why are US car workers on strike?

The UAW represents more than 140,000 workers at GM, Ford and Stellantis.

In an update to union members last week, UAW boss Shawn Fain said negotiations were making progress but he believed there was "more to be won".

He urged workers to remain steadfast as the talks continue, telling them: "We've got cards left to play and they've got money left to spend."

The costs from the strike weighed on GM's profits,which came in at $3.1bn over the July-to-September period, down about 7% from the same quarter a year ago.

However, the fall was not as steep as analysts had expected, and revenue was up more than 5% from last year to $44.1bn.

GM said it was working on trimming costs by $2bn.

It said it was postponing some of its plant openings, in part reflecting slower-than-expected sales of electric cars. It said it still hoped to make one million electric cars in the US by 2025.