WASHINGTON--A nuclear-powered Soviet submarine appears to have sunk more than 300 miles off the coast of Norway after suffering an underwater accident and catching fire, U.S. and Norwegian officials said yesterday.

U.S. officials said there were "definitely" casualties on board the stricken submarine but declined to estimate how many.

They said the submarine managed to surface for a time in the Norwegian Sea after the accident occurred and that at least one Soviet ship had responded to a distress call and arrived on the scene.

"It was on the surface, but now it appears it's gone," said one official, speaking on condition of anonymity. "We're trying to get confirmation, but we believe it sunk."

White House Press Secretary Marlin Fitzwater said the accident appeared to involve a Soviet "Mike" submarine--one of the Russian Navy's newer attack subs designed to hunt and destroy other submarines in the event of war.

"There are indications there was a major fire aboard, but we have no details. We understand there has been a loss of life, but we can't confirm how many," Fitzwater said. "We express the sympathy of the United States government to the Soviet government for any loss of life involved."

One U.S. source placed the last known position of the vessel about 310 miles to the north of Norway's North Cape.

In Oslo, a Norwegian army supreme command spokesperson said the sub apparently was lost in the area between Norway and Greenland. Col. Gollow Gjeseth, press spokesperson of Norway's Supreme Defense Command, said in a telephone interview that a Norwegian observation plane sent over the area about 11 a.m. EDT saw "a Soviet ship and two rubber boats."

NTB, the national news agency, said the plane saw several Soviet planes and vessels, apparently part of a Soviet rescue operation in the area, which is in international waters.

According to the authoritative reference book Jane's Fighting Ships, the Soviets have so far launched only one ship in the "Mike" class.

That vessel was launched in May 1983 and went into service in late 1984, according to Jane's. The Pentagon reference book Soviet Military Power says the Mike vessel was not declared operational until 1986.

According to Jane's, the vessel is powered by two nuclear reactors and is believed to carry a crew of about 95.

The proper Russian name for the submarine was not immediately available. The name "Mike" is a U.S. and NATO nickname for the sub.

According to Jane's, the Mike submarine's reactors are described as "probably liquid metal cooled," an approach that "allows for a saving in space as well as an increase in power."

The sources stressed, however, it was unclear whether the nuclear propulsion section of the Russian sub had been the source of the accident or involved in any fires.

The Mike submarine is described as being about 361 feet long and capable of steaming at 38 knots when submerged. The sub displaces about 6400 tons when submerged.

Jane's and other Pentagon reference books describe the Mike as a submarine that "seems to be a test vehicle for new design and propulsion features."

Administration sources said U.S. intelligence agencies have long assumed the Mike sub was built to serve as a "test platform" for new types of submarine-launched cruise missiles.

The accident with the Mike comes two-and-a-quarter-years after a Soviet "Yankee-class" missile submarine sank off the coast of North Carolina after an explosion and fire. That fire was believed to have involved the liquid fuel propellant inside a nuclear missile.

According to William Arkin, a specialist on nuclear weaponry with the liberal think tank Institute for Policy Studies, the Soviet navy has suffered more than 200 serious accidents with submarines just since 1975.