"There is much to indicate that the number of dead was higher than first thought - though we do not have an exact figure," Defense Ministry spokesman Erik Senstad told Reuters. Earlier Western estimates put the number of known dead at 12 and about 50 missing.

Senstad said Soviet authorities had supplied additional information about the disaster and confirmed that the vessel was a Mike class submarine, the only one of its type and used to test advanced weapons systems.

"We know now that there was a powerful explosion on board after the fire spread on and the vessel began to list," he added.

Soviet leader Mikhail Gorbachev said the submarine's nuclear reactor was shut down before it sank. But no casualty figures have been given.

The submarine would normally carry a crew of 95, and a Norwegian Defense Ministry statement said it seemed that between 40 and 50 people appeared to have escaped before it sank in 5,000 feet of water.

"How many of those survived injuries and exposure to the cold water is uncertain," it added. The ministry said a Norwegian air force observation plane had spotted men clinging to life rafts.

Col. Gullow Gjeseth of Norway's Defense Command said of the television death toll report: "It is possible that the figure could be correct but we cannot say. What we know is that the figure is probably considerably higher than 12. But the Russians have not given us a body count."

In Washington, the White House said President Bush received a message from Gorbachev confirming that the submarine had sunk and ruling out the possibility of an explosion and radioactivity.

The message was sent from the Soviet Embassy to the State Department and passed on to Bush.

Fire swept through the submarine just before 2 a.m. MDT Friday. The vessel sent a distress signal 45 minutes later which brought Soviet aircraft and ships rushing to the scene. It sank about five hours later.

"During the morning, the fire spread to more sections. An explosion followed and the submarine took a list," the Defense Ministry said. "Crew members who had survived probably left the submarine when it began to sink. It must be assumed that the reactor at this time was shut down. Soviet authorities . . . confirmed that the submarine went down at 1515 hours local," it said.

Soviet Defense Minister Dmitry Yazov said earlier Saturday that the crew fought the blaze for several hours before the submarine sank. He said rescued crew members were on board a Soviet warship heading for the Soviet port of Murmansk.

All that remained by Saturday was a small oil slick on the surface of the water, although Soviet vessels spent the day searching for survivors.

Norwegian scientists have taken water samples to run a series of checks, despite a personal assurance from Gorbachev to Norway's Prime Minister Gro Harlem Brundtland that there was no danger of a reactor meltdown or a radiation leak.

"The reactor has been shut down, and according to expert assessments the possibility of an explosion and radioactive pollution of the environment is excluded," Gorbachev said.

Gorbachev, who had just returned from Britain, sent a similar message to Prime Minister Margaret Thatcher. His messages seemed designed to calm Western public opinion, still highly sensitive to nuclear accidents after the Chernobyl disaster three years ago.

Soviet officials have not said what caused the fire. Gorbachev said the submarine was armed with torpedoes, but he did not make clear whether they were nuclear or conventional. Norwegian sources said they were probably nuclear.

U.S. naval expert Richard Fieldhouse, who has published a study of the Soviet navy, said the Mike submarine launched in 1983 was the only one of its kind and was used as a platform on which to test advanced weapons systems.

"They devise new systems, the weapons technologies of the future, and test them on the Mike . . . If they have lost this vessel, that will be a serious blow to the prestige of the Soviet Navy and may well have political repercussions," he said.

According to the authoritative Jane's Fighting Ships, the 360-foot submarine had a crew of 95 and could carry cruise missiles and anti-submarine missiles.

Another U.S. expert said the type of nuclear reactors aboard the vessel was inherently dangerous.

"This submarine carried two liquid metal-cooled reactors. You are literally using molten metal," said William Arkin, a specialist on nuclear weapons with the Institute of Policy Studies in Washington.

"It's dangerous and difficult to operate. We tried it once and gave up because it's too volatile," he told Britain's Guardian newspaper.