WEST BRIDGEWATER, Mass. – Negotiators for Shaw’s Supermarkets and the union representing 6,000 grocery workers agreed Tuesday to extend their expired contract by several days in hopes of avoiding a strike.

The two sides went back to the bargaining table Tuesday, two days after members of United Food and Commercial Workers Local 791 voted overwhelmingly to authorize a strike. Peter Derouen, a spokesman for the union, said negotiators agreed to extend the previous contract to midnight Thursday.

The contract expired at midnight Saturday, but employees have continued working under its terms while negotiators try to hammer out a new agreement.

The two sides met with a federal mediator for 14 hours Monday at an undisclosed location. The talks resumed Tuesday morning.

UFCW members voted overwhelmingly Sunday to turn down a contract Shaw’s had presented hours earlier and characterized as its final offer.

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. Shaw’s stores in Maine are nonunionized and are not directly affected by the labor talks.

“There’s a lot of issues out there,” Derouen said, “and we want to make sure that everything is handled.”

Shaw’s spokesman Terry Donilon characterized the tone of the talks as “professional.”

Negotiators are trying to resolve differences over issues including health care costs, wages, work rules and pension benefits.

West Bridgewater-based Shaw’s has called the offer it presented Sunday very generous, but the union said it 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.

Shaw’s is owned by Boise, Idaho-based Albertsons Inc., one of the nation’s largest food and drug retailers.