The United Automobile Workers union increased the pressure on Ford Motor and General Motors by extending its strike to two more car assembly plants on Friday, saying the companies had not moved far enough to meet its demands for higher pay and benefits.

The move is the second escalation of strikes that started on Sept. 15 at three plants, one each owned by G.M., Ford and Stellantis, the parent of Chrysler, Jeep and Ram. The union said it would not expand the strike against Stellantis this week because of progress in negotiations there.

The U.A.W.’s president, Shawn Fain, said workers at a Ford plant in Chicago and a G.M. factory in Lansing, Mich., would walk off the job on Friday. G.M. makes the Buick Enclave and Chevrolet Traverse sport-utility vehicles at the Lansing plant. Ford makes the Explorer, the Police Interceptor Utility and Lincoln Aviator in Chicago.

“Ford and G.M. have refused to make meaningful progress at the bargaining table,” Mr. Fain said in a live-streamed video.

Ford’s Chicago plant employs about 4,600 U.A.W. members and G.M.’s Lansing plant has 2,300 union workers. Including the workers who walked off the job earlier, more than 25,000 U.A.W. members at the three companies have been called on to stop working.

A week ago, workers walked out at 38 spare-parts distribution centers owned by G.M. and Stellantis. The U.A.W. did not expand its strike at Ford because, the union said at the time, it had significant progress in contract negotiations with that company.

The U.A.W. is seeking a substantial wage increase for workers and opened the talks by demanding a 40 percent raise, pointing to the substantial profits all three companies have generated over the last decade and to the size of the pay increases they have given their chief executives over the last four years.

The companies have each offered roughly 20 percent over four years. Ford and the union have reached agreements on some other demands, including cost-of-living adjustments if inflation surges again and the right to strike if the company closes plants.

The parties have met regularly, and on Thursday the union presented its latest counteroffer to Stellantis, the union said. Negotiating teams from the U.A.W. and G.M. met on Wednesday in a session attended by Mr. Fain.

The union leader’s online remarks on Friday were delayed for nearly half an hour by what he called “a flurry of interest from the companies in addressing some serious bargaining issue.” He did not provide more details.

A G.M. executive told employees on Friday that the company was waiting for a “comprehensive counteroffer” from the union in response to an offer it made on Sept. 21.

“Calling more strikes is just for the headlines, not real progress,” Gerald Johnson, executive vice president for global manufacturing, said. “The number of people negatively impacted by these strikes is growing and includes our customers who buy and love the products we build.”

Ford scheduled a 1 p.m. conference call with reporters to respond to the U.A.W.

Tensions on the picket lines have flared this week. The union said five strikers on the picket line suffered minor injuries when they were hit by a car outside a G.M. plant in Flint, Mich. Other confrontations occurred at picket lines in California, Massachusetts and Michigan, the union said.

On Friday, Mr. Fain is scheduled to appear at a rally at the union’s headquarters in Detroit. Two union locals involved in the strike have called on workers to form a convoy of vehicles made in the idled plants stop at the rally as part of a drive from Toledo, Ohio, to Wayne, Mich., the locations of struck Jeep and Ford factories.

“We will not be intimidated into backing down,” said Mr. Fain, who has frequently compared the strike to a “war on corporate greed.”

In a statement on Thursday, Stellantis criticized Mr. Fain’s characterization of the negotiations, and blamed the union for violence, saying that some strikers had slashed tires on trucks and harassed nonstriking employees at parts warehouses.

“The deliberate use of inflammatory and violent rhetoric is dangerous and needs to stop,” Stellantis said. “The companies are not ‘the enemy’ and we are not at ‘war.’ We respect our employees’ right to advocate for their position, including their right to peacefully picket. But the violence must stop.”

The strategy of striking at only a limited number of locations, but spreading the walkouts to plants owned by all three automakers, is a break from U.A.W.’s traditional approach of idling most or all operations at one company. In 2019, union workers went on strike at G.M. for 40 days before a tentative agreement was reached.

Mr. Fain has said the new strategy is intended to keep the companies guessing about what parts of their operations would be hit next, in hopes of improving the union’s negotiating position. The first three plants hit by the strike make some of the automakers’ most profitable vehicles, including the Chevrolet Colorado, Ford Bronco and Jeep Wrangler.

A limited strike also dents the companies’ profits while limiting damage to their suppliers, local businesses and the national economy.

Expanding the strike also increases the financial cost to the union. It is paying striking workers $500 a week out of its $825 million strike fund.

Santul Nerkar contributed reporting.