The US auto workers union expanded on Friday a strike against two of Detroit's "Big Three" automakers and invited President Joe Biden to support workers on the picket line.

UAW President Shawn Fain announced a strike of all 38 US parts and distribution centers at General Motors and Stellantis, where negotiations are stalled. He did not expand the stoppage at Ford, which has offered important concessions since the strike was launched a week ago.

"As we have said for weeks, we're not going to wait around forever for fair contracts at the Big Three," Fain stressed in a briefing.

He continued, "We invite and encourage anyone who supports our cause to join us on the picket line, from friends and family all the way up to the president of the United States."

"The way you can help is to build our movement and show the companies that the public stands with us," Fain indicated.

The UAW President pointed out that Ford had improved earlier proposals by reinstating a cost-of-living measure that had been suspended in 2009. Ford also offered an enhanced profit-sharing system and granted the union the right to strike over plant closures.

"We're not done at Ford," Fain said, but "we do recognize that Ford is serious about reaching a deal."

"At GM and Stellantis, it's a different story."

The UAW's strategy of gradually expanding its action is part of what Fain has dubbed the "stand-up strike" that aims to maximize the union's bargaining leverage due to the risk that additional plants will be taken down.

Under Fain, the UAW has adopted an aggressive posture in the talks, accusing the companies of "corporate greed" and lambasting the Big Three CEOs' salaries of more than $20 million each.

Fain has also put aside the UAW's convention of picking one of the three companies as a strike target, instead launching three independent series of talks with the companies.

The UAW is seeking 40% wage hikes that would match the average increases by CEOs over the last four years.

Other key demands include the elimination of different worker pay and benefit "tiers", a cost-of-living adjustment, and the re-establishment of retiree medical benefits and a pension for junior employees.

The one-week-old strike has so far had a limited effect on company profits while introducing new pressures on auto parts suppliers to the Big Three.