The US auto workers union expanded a potentially economically and politically damaging strike against two of Detroit's "Big Three" Friday – 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 centres at General Motors and Stellantis, where negotiations are stalled.

Fain did not expand the stoppage at Ford, where there are still significant gaps, but 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 said in a briefing.

"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," Fain said – an invitation that will further push the White House into the politically treacherous issue.

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

Fain said 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 granting the union the right to strike over plant closures.

"We're not done at Ford," Fain said.

However, "we do recognise that Ford is serious about reaching a deal," he said. "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" – an allusion to the UAW's landmark "sitdown" strike of the 1930s – that aims to maximise the union's bargaining leverage due to the risk 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 Big Three CEO salaries of more than US$20 million each.

Fain has also shelved the UAW's convention of picking of one the three companies as a strike target, instead launching three independent series of strikes that have kept the companies off guard.

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

Other key demands include an elimination of different worker pay and benefit "tiers," a cost-of-living adjustment, and the reestablishment 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. But the expanded strike will pose new challenges for Stellantis and GM.

Analysts consider the worst-case economic scenario to be a lengthy strike that pinches consumption from workers who are taking home US$500 in weekly strike pay instead of their normal wages. (AFP)