CHICAGO, Oct. 24 (Xinhua) -- The United Auto Workers (UAW) spread its strike against the Big Three U.S. automakers to Stellantis NV's Sterling Heights Assembly Plant in the U.S. state of Michigan on Monday.

About 6,800 UAW member workers at the plant build popular RAM 1500 truck. The plant is Stellantis' largest plant as well as the biggest moneymaker, local media reported.

"Stellantis has the worst proposal on the table regarding wage progression, temporary worker pay and conversion to full-time, cost-of-living adjustments, and more," the UAW said in a statement.

"We are outraged that the UAW has chosen to expand its strike action against Stellantis," local media quoted Stellantis as saying in a response statement on Monday.

Stellantis on Thursday morning presented a new, improved offer to the UAW, including 23-percent wage increases over the life of the contract, nearly a 50-percent increase in contributions to the retirement savings plan, and additional job security protections for employees, the statement said.

"Following multiple conversations that appeared to be productive, we left the bargaining table expecting a counter-proposal," the automaker said in the statement. "Our very strong offer would address member demands and provide immediate financial gains for our employees. Instead, the UAW has decided to cause further harm to the entire automotive industry as well as our local, state and national economies."

UAW President Shawn Fain said earlier that the union has changed it strategy and would call on plants to join the strike at any time, not just Fridays.

Fain on Friday detailed the current proposals from General Motors (GM) Co., Ford Motor Co. and Stellantis, when he highlighted the shortcomings of Stellantis' current offer. All Big Three now have offered 23-percent wage increases and agreed to eliminate wage tiers.

The UAW announced a strike at three select factories of Ford, GM and Stellantis on Sept. 14, after its contract with the Big Three expired.

The union spread the strike to 38 GM and Stellantis parts distribution centers on Sept. 22, to GM and Ford SUV assembly plants on Sept. 29, and to Ford's highly profitable truck plant in Kentucky on Oct. 11.

The UAW strike now covers the Big Three's seven assembly plants and 38 parts distribution centers in 22 states.

To date, about 40,000 out of some 146,000 Big Three U.S. automakers represented by the UAW are on strike across the country. With nearly 4,000 Mack Trucks workers going on strike on Oct. 9, the total number of UAW members on strike now exceeds 44,000.