BOSTON — Firefighters braving “incredible heat and smoke” extinguished a fire that broke out aboard a U.S. Navy nuclear-powered submarine docked in Maine and caused extensive damage, a Navy official said on Thursday.

Seven people were injured putting out the fire that broke out on Wednesday night on the USS Miami, docked at the Portsmouth Naval Shipyard in Kittery, but the blaze at no time threatened the ship's nuclear facilities, Rear Admiral Rick Breckenridge said at a briefing.

“The fire spread to spaces within the submarine that were difficult to access presenting a challenging situation for initial responders - but they persevered in incredible heat and smoke,” Breckenridge said.

“A full investigation has begun to determine the cause of the fire. This investigation will be thorough and will take some time to get it right,” he said.

Damage was “extensive,” but limited to the forward compartment spaces, which include crew living and command and control spaces, Breckenridge said.

It took the Portsmouth Naval Shipyard Fire Department and Ship's force, along with several area fire departments, more than 12 hours to extinguish the fire.

“The heat, smoke and close quarters made this a very dangerous response,” said Maine Governor Paul LePage.

Breckenridge, who is in charge of submarines in the region, said the ship's nuclear reactor had been shut down for more than two months and remained in safe and stable condition throughout the event. There were no weapons on board in the torpedo room, he said.

The USS Miami, worth an estimated $900 million, was in the third month of a planned 20 months of maintenance at the Portsmouth Naval Shipyard.

The submarine, whose home port is Groton, Conn., arrived at the shipyard in March. The submarine typically carries a crew of 13 officers and 120 enlisted personnel.

The injured personnel included three Portsmouth Naval Shipyard fire-fighters, two crew members and two civilian fire-fighters providing support. They were either treated on the scene or taken to a local medical facility. All have been released.