(Bloomberg) -- General Motors Co. and the United Auto Workers are bargaining over a pair of proposals, one made late Friday by the union and the other by management last week, in an attempt to break an impasse and end a four-week-old strike.

The two sides didn’t immediately come to an agreement Saturday following the union’s proposal, which was presented after two days of public bickering in which GM and UAW leaders accused each other of dragging out the strike. The union has said that job security remains a sticking point in talks.

“The two sides are probably bargaining over the union’s proposal,” said Harley Shaiken, a professor of labor at the University of California at Berkeley. “I suspect that was understood from both sides that the union would make its proposal and then they would bargain back and forth.”

The UAW’s walkout has halted production at 34 U.S. plants and disrupted output at factories in Mexico and Canada. Analysts say the strike has cost GM more than $1 billion in lost profit.

On Monday, GM made a proposal that includes 3% pay raises for workers in years two and four of the contract, and lump sum payments of 3% and 4% in the other years, according to people familiar with the matter. Full-time workers would be offered a $9,000 signing bonus, which is $1,000 more than they got in the 2015 contract, and temporary staff would get $3,000 for ratifying the deal, said the people, who asked not to be named because negotiations are private.

GM also boosted its planned investment in U.S. factories to $9 billion from its initial offer of $7 billion on Sept. 14.

For temporary workers, GM would offer a path to full-time employment after three consecutive years of work.

That part of the offer is likely to be contentious for the union, which wants workers to be able to gain full-time status after three total years of employment or less, Shaiken said. Otherwise, the union thinks GM would lay workers off before the three-year term has ended and just hire new temps to avoid adding permanent staff, he said.

In addition, GM would keep the health care plan intact, which requires union workers to pay just 3% of their medical costs.

The talks have been going on for several months. The treatment of temporary workers was a key bargaining point since 3,000 of GM’s 46,000 hourly employees are temps. Since many have been temps for as long as four years making a little more than half the top assembly wage of $30 an hour, the union has sought a path to full-time work and better pay.

The union also wants GM to allocate more future models to U.S. plants to safeguard against layoffs. When GM idled four plants, including a large compact car plant in Lordstown, Ohio, earlier this year, the union was outraged.

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