A fire on a nuclear-powered submarine at a Maine shipyard has injured four people.

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

The shipyard says four injured people have been treated and released. The Portsmouth Herald newspaper said firefighters were injured.

Shipyard public affairs specialist Gary Hildreth said the fire started in the forward compartment of the sub. The shipyard said the sub's reactor was not operating at the time and was unaffected. The cause of the fire has not been identified.

Non-essential personnel were removed from the sub. Black smoke billowed overhead.

The USS Miami arrived at the shipyard for maintenance and upgrade work in March.