By David Shepardson, Joseph White

DETROIT (Reuters) -The United Auto Workers union could strike additional Detroit Three automotive facilities on Friday if there is no serious progress in labor negotiations, a source familiar with the talks said.

The union did not elaborate ahead of a planned video address by UAW President Shawn Fain at 10 a.m. ET (1400 GMT) on Friday. Bargaining continues at Ford, General Motors and Chrysler parent Stellantis.

If Fain triggers walkouts at more plants starting at noon (1600 GMT) on Friday, the UAW is expected to continue work stoppages currently underway until a new contract is ratified, the source said, speaking on condition of anonymity.

The union has already shut down one assembly plant at each of the Detroit Three, and 38 parts distribution centers at GM and Stellantis.

About 18,300 UAW members are on strike, but that is only about 12% of the total number of union members working at the three automakers. Strikers are getting $500 a week from the UAW's strike fund.

The UAW launched its walkouts on Sept. 15, its first simultaneous strikes at General Motors, Chrysler parent Stellantis and Ford. The union on Sept. 22 expanded its strikes against GM and Stellantis, but kept its Ford walkout limited to a single plant. It is unclear whether Ford will be targeted in the next round of actions.

So far, the union and the companies remain far apart on key economic issues. Fain has stuck with a demand for 40% pay hikes over a four-year contract, a position supported by President Joe Biden during a visit to Detroit on Tuesday. The companies have countered with offers of about 20%.

The UAW also is pushing automakers to eliminate the two-tier wage system, under which new hires can earn far less than veterans.

The union has said negotiators at Ford are making the most progress toward an agreement. But Ford has said wide differences remain over key issues.

The UAW and GM plan talks on Wednesday afternoon, two sources familiar with the discussions said.

GM, Ford and Stellantis were not immediately available for comment.

The UAW's list of potential next strike targets includes engine and transmission facilities, as well as factories producing large pickup trucks and SUVs.

A wider strike that shuts down production of large trucks and SUVs could cost the automakers billions of dollars in revenue and profit. Analysts estimate GM, Ford and Stellantis earn as much as $15,000 per vehicle on each of their respective large pickup truck models.

The UAW, which represents 46,000 workers at GM, 57,000 at Ford and 43,000 at Stellantis, began negotiations with the companies in July.

Along with Biden, other politicians from both the Democratic and Republican parties have expressed support for the UAW's drive for higher pay.

Former President Donald Trump, the Republican frontrunner for the 2024 nomination, will speak in Michigan on Wednesday evening at a non-union auto manufacturer, as he skips the second Republican U.S. presidential debate. The UAW is not involved with that visit, and Fain has been critical of Trump in recent days.

The visits by Biden and Trump highlight the importance of union support in the 2024 presidential election, even though union members represent a tiny fraction of U.S. workers.

(Reporting by David Shepardson and Joseph White; Editing by Leslie Adler and Jonathan Oatis)