U.S. new vehicle sales likely rose in the third quarter on improving supply and steady demand, but a strike by autoworkers has muddied the outlook for the "Detroit Three" for the rest of the year, analysts said.

General Motors was set to remain the top-selling automaker with 680,009 vehicles sold in the quarter ended September, while second-placed Toyota was expected to finish the quarter at 583,661 vehicle sales, according to car shopping website Edmunds.

But that may change after the United Auto Workers (UAW) union launched unprecedented, coordinated strikes against GM, Ford and Stellantis last month.

"The production disruptions caused by the strike will have ramifications for potential sales levels moving through the fourth quarter," S&P Global Mobility said in a note.

The UAW union initially targeted one assembly plant at each of the Detroit Three automakers that make some of their most profitable models and later expanded the coordinated strike, citing a lack of progress in talks.

S&P forecast daily losses from the strike at the three plants at over 4,000 units, warning further walkouts may lead to cumulative losses reaching hundreds of thousands of units.

The automakers haven't disclosed a financial hit from the strikes so far and September sales are likely to be unscathed given the inventory built up in anticipation of the strike. But analysts have warned that a prolonged, full strike may cost billions of dollars.

That may play into the hands of non-unionized automakers such as Tesla and foreign brands like Toyota, with the companies grabbing market share from the Detroit Three.

Meanwhile, pent-up demand from retail and commercial customers could buoy third-quarter sales.

U.S. new vehicle sales likely rose 17% to 3,941,700 units, whereas, the September seasonally adjusted annualized rate is expected to be 15.4 million units, consultants J.D. Power and GlobalData said.

Automakers are expected to report quarterly sales in the coming days.

(Reporting by Shivansh Tiwary in Bengaluru; Editing by Shounak Dasgupta)