Chrysler parent Stellantis has sharply criticised the demands of the United Auto Workers (UAW) union, saying they need "a focus on reality from everyone involved".

Stellantis North America chief operating officer Mark Stewart said in a letter to employees seen by Reuters the automaker was "committed to working with the UAW to reach an agreement based on economic realism".

A UAW spokesman declined immediate comment to the news agency on Stewart's letter but earlier this week UAW President Shawn Fain called the Stellantis proposals "trash" and tossed a copy of them in a waste basket in live streamed remarks.

Stewart reportedly added agreeing to Fain's "demands could endanger our ability to make decisions in the future that provide job security for our employees. This is a losing proposition for all of us".

Reuters noted the current four year contracts with Stellantis, General Motors and Ford expire on 14 September.

The UAW had said it was seeking "audacious and ambitious" improvements, including pay raises of more than 40% over four years, significant additional time off, and a restoration of defined benefit pensions previously eliminated for newer workers.

Fain criticised numerous concessions Stellantis was seeking.

"Stellantis proposals are a slap in the face," Fain said, disclosing the company was proposing cuts to healthcare coverage, fewer vacation days for new hires and lifting a cap on temporary employees.

Stewart said Fain did not fairly represent the negotiations.

"The theatrics and personal insults will not help us reach an agreement," Stewart wrote, adding "now is the time to come to the table with open minds and a commonsense approach." He added "at this very early stage, no one should jump to any conclusions about the outcome of the process."

Reuters noted Stellantis had made proposals aimed at reducing absenteeism and cutting pension, healthcare and other costs, saying that amid government electric vehicle rules, it was imperative to "find ways to reduce the overall fixed cost structure of our business".

The UAW also reportedly said the company opposes an end to two tier wages, a practice of newer hires getting paid much less than veteran workers.

Two people briefed on the matter told Reuters automakers had estimated the UAW's contract demands could raise the current mid-$60-per-hour labour rate to more than $150 per hour.