The Department of Defense has issued an $85 million contract modification for work at Bath Iron Works.

The contract modification released Tuesday by the U.S. Naval Sea Systems Command is for the design, planning and material support services for maintenance and modernization of DDG-51 Arleigh Burke-class destroyers and guided missile frigate ships.

The yard is currently working on six destroyers – three Zumwalt-class ships and three Arleigh Burkes – which are at various stages of construction. It also performs work on frigates, known as FFG-7 class vessels, but no longer builds the ships in Bath.

The additional design and planning work is expected to be completed by December 2016.

The original contract was awarded in 2012, according to BIW spokesman Matt Wickenheiser. The yard is in the fifth year of a five-year contract.