BOSTON (AP) – Shaw’s Supermarkets and union negotiators representing 6,000 grocery workers in three New England states returned to the bargaining table Monday a day after union members rejected a contract offer and authorized a strike.

Negotiators for both sides met with a federal mediator at an undisclosed location in a session that begin around 9 a.m. and was continuing into the evening.

Members of United Food and Commercial Workers Local 791 overwhelmingly voted Sunday to turn down a contract Shaw’s had presented hours earlier and characterized as its final offer. Health care is the key sticking point. They also authorized a strike by workers at 25 stores in southeastern Massachusetts, all 14 Shaw’s stores in Rhode Island and at a distribution center in Wells, Maine.

Employees continued working at the affected Shaw’s stores Monday, though their contract expired Saturday night.

Workers at other Shaw’s stores in Massachusetts, Maine, New Hampshire and Vermont are non-unionized and unaffected by the labor talks, and unionized Shaw’s employees in Connecticut are covered by a different local.

Shaw’s spokesman Terry Donilon said Monday the company was prepared “to meet as long as it takes to reach an agreement.” He said the company was ready to continue operating stores that would be affected by a strike, but offered no details on how that would be accomplished.

Local 791 spokesman Peter Derouen said the union was focused on trying to reach an agreement but was prepared to strike and had picket signs ready to go.

“It’s up to the company now to get serious about negotiations,” he said.

West Bridgewater-based Shaw’s has called its offer very generous and said it would make its unionized workers “among the best compensated in the industry in New England.” The company said its latest offer includes wage increases in each year of the contract, pension increases and a choice of four health plans that would cover 85 percent to 90 percent of full-time employees’ medical costs.

But Derouen said the proposal offered few improvements from an earlier version. Talks have been under way since June, with the company’s plans to control rising health care costs emerging as the main sticking point. Other differences remain over wages, work rules and pension benefits.

Shaw’s, owned by Boise, Idaho-based Albertsons Inc., one of the nation’s largest food and drug retailers, says costs for the union’s health care plan for the 39 stores have risen more than 60 percent in the last three years.

The union has said the company’s proposal would reduce benefits and shift thousands of dollars to employee insurance copays without increasing wages.