The union representing striking workers at Bath Iron Works has requested the assistance of a federal mediator, an official said Thursday.

The request for a mediator’s help was made by the International Association of Machinists and Aerospace Workers Local S6 on Tuesday, but the company would have to agree to mediation, said Jay Wadleigh, a district business representative for the Machinists union. There was no immediate comment from the company.

Members of Bath Iron Works’ biggest union, the 4,000-member organization went on strike on Monday after voting overwhelmingly to reject the company’s final contract offer, citing disagreements about senior preferences, subcontracting and health care premiums, among other issues.

The strike comes while BIW, one of Maine’s largest private employers, is already six months behind on producing a set of destroyers. It is the first strike at the shipyard in 20 years.

The issues of contention have more to do with subcontractors, work rules and seniority than wages and benefits. The company’s three-year proposal would have given shipbuilders annual 3 percent pay raises.

The shipyard is one of the Navy’s five largest shipbuilders and a major employer in Maine with 6,800 workers.