KITTERY, Maine â€"  A fire on a nuclear-powered submarine at a Maine shipyard has injured six people, including a firefighter.

Fire crews responded Wednesday to the USS Miami SSN 755 at the Portsmouth Naval Shipyard on an island in Kittery, a town near Portsmouth, N.H., popular with tourists.

The fire, which began at around 5:45 p.m., was still burning early Thursday morning but shipyard spokesman Capt. Bryant Fuller said in a statement that the situation was improving.

The shipyard said the injured people were treated and released. The firefighter suffered heat exhaustion.

Shipyard public affairs specialist Gary Hildreth said the fire started in the forward compartment of the sub. The shipyard said the sub's reactor wasn't operating at the time and was unaffected. There also were no weapons on board.

The cause of the fire hasn't been identified.

Nonessential personnel were removed from the sub. Black smoke billowed overhead.

The USS Miami arrived at the shipyard for maintenance and upgrade work in March. Its home station is Groton, Conn., where the U.S. Navy has a base.