Fire-damaged U.S. nuclear submarine USS Miami may prove too expensive to repair with no certainty of its future safety, defense analysts said.

U.S. Navy estimates for the vessel's repair and refitting run to $400 million estimates, the Portland (Maine) Press Herald said as military experts examined burned sections of the sub.

Defense shipbuilding industry experts said the estimate was a wake-up call for the U.S. defense engineering and shipbuilding industry, which needs to be more competitive.

The 360-foot USS Miami burned May 24. At least seven people, two crew members and five firefighters, suffered minor injuries in the fire on the sub while it was in dry dock at Portsmouth Naval Shipyard in Kittery, Maine.

The Navy said the fire apparently started in a vacuum cleaner on the boat to clean areas where repairs were being done.

Navy officials issued the preliminary cost estimate of $400 million to repair the nuclear submarine -- if it can be fixed at all, the Maine newspaper said.

It said navy investigators were examining the submarine to determine the extent of the damage and whether the vessel could be saved. The Navy must be sure that the steel hull wasn't damaged to the point that it cannot withstand the intense pressure of the deep ocean, the newspaper said in a report on its Web site, pressherald.com.

Fire broke out in the forward compartments of the submarine May 23 and burned for 10 hours as fire crews from three states fought to extinguish it.

Members of Maine's congressional delegation said the shipyard is in a good position to make the repairs if the Navy decides to go ahead.

However, the $400 million initial estimate could change, U.S. Rep. Chellie Pingree, D-Maine, who toured the submarine this week, warned. She called the estimate "a workable figure."

The navy's estimate meant that "the boat is repairable, that the work can be done at Kittery and the Navy will be able to come up with the money for the repair," said Pingree, a member of the U.S. House of Representatives Armed Services Committee.

U.S. Sen. Susan Collins, R-Maine, a member of the Senate Armed Services Committee, said more money could be made available through a defense appropriations subcommittee in the Senate.

"Should the Navy determine ... that the Miami can safely operate following repairs, I stand ready to work to help ensure the Navy has the necessary funds," said Collins, in comments cited by the newspaper.

The $400 million repair estimate is twice an earlier estimate for the vessel's scheduled overhaul, which was intended to extend submarine's life by 10 years. It isn't clear if the $400 million estimate is in addition to the $200 million already budgeted for the overhaul.

The Miami is a Los Angeles class attack submarine that cost $900 million to build in 1990. It was commissioned in June 1990. The latest Virginia class submarine costs about $2 billion to build.

The repair costs could have been higher but the submarine was being emptied of expensive equipment for the overhaul when the blaze struck. The rear half of the sub, where the nuclear propulsion system is located, escaped the flames.

There were no weapons aboard, officials said.

The USS Miami, with a crew of 13 officers and 120 enlisted personnel, arrived at the shipyard March 1 for maintenance work and system upgrades.

The number of U.S. attack submarines is set to drop in the coming years as older submarines are retired more quickly than they are replaced. Against 98 submarines in the late 1980s, the U.S. Navy will have 43 in 2020. Experts have recommended a minimum of 55 submarines for the U.S. armed forces.