An offshore oil-drilling platform being used as a "floating hotel" for Phillips Petroleum workers overturned in the middle of the North Sea last night, killing at least seven men and leaving 108 others missing at sea or trapped inside the submerged platform.

Despite gale-force winds, waves, darkness and fog, Norwegian and British helicopters and boats rescued 110 oilworkers who escaped from the platform as it fell into the sea. Poor visibility and fatigue forced helicopter pilots to suspend rescue efforts about 2 a.m.

At dawn, Norwegian rescuers sent divers in three diving bells to search for the trapped men, including about 50 believed to have been inside the platform's movie theater. But officials in Stavanger, the North Sea oil industry headquarters town on the southwestern coast of Norway, held out less and less hope for the survival of large numbers of men in the icy waters.

It is the worst accident in a decade of drilling for oil in the North Sea, and the first time that one of the dozens of giant floating platforms has capsized.

An estimated 225 oil workers -- nearly all Norwegian, although some could have been American or British -- were aboard the French-built, Norwegian-owned platform, called the Alexander Kielland, in the Ekofisk oil field just inside the Norwegian oil exploration sector near the center of the North Sea, 240 miles away from the nearest land.

The platform, which had been converted from a drilling rig and still had a derrick on it in addition to living quarters, served as overflow accomodation for the Phillips' Edda production platform 300 yards away.

At about 6:30 p.m. yesterday, according to a Phillips spokesman in Stavanger, one of the pentagon-shaped platform's five submerged pontoon legs broke off without warning. The platform first tilted to one side and then "overturned pretty quickly," according to the spokesman, who said "it is now floating upside down" with the living quarters completely under water. Only its remaining air-filled legs were sticking up out of the sea.

As the platform fell over, many of those aboard fell or jumped into the sea, some with lifeboats, rafts or lifebelts. A few were able to make it by themselves to the nearby production platform, which was unharmed.Others were rescued one at a time by winches from rescue helicopters operating from other Ekofisk platforms. And 28 men climbed aboard a large rescue raft lit by beam and flares from a British Royal Air Force Nimrod aircraft.

The rescued workers were being treated in medical facilities on many of the 22 platforms in the Ekofisk field, according to a Phillips spokesman. The seven bodies were recovered near the platform.

Although there were gale-force winds and 20-foot waves when the platform overturned, conditions were not bad enough to break one of the rig's steel pontoon legs, the spokesman said. The platform was built to withstand 80-foot waves. A spokesman for the firm that insured the rig said in Oslo that it had been checked for safety last September and was due for a complete overhaul in July. The platform was leased to Phillips by Stavanger Drilling, a Norwegian firm.

Early today, the wind fell and the sea becalmed but low cloud cover and fog made it impossible for rescuers in the helicopters to see the water with searchlights from just 50 feet above it and the flights were stopped until daylight. Private helicopters that link the oil platforms with each other and the mainland had been joined in the search by British and Norwegian military craft.

The rough weather hampered rescue efforts, according to a British RAF spokesman. "The conditions were pretty atrocious to begin with," he said, "and they gradually got worse. Our crews became exhausted and their efforts extremely hazardous."

By 5 a.m., most of the survivors had been flown to hospitals in Stavanger.

Phillips Oil of Norway, a subsidiary of the American firm headquartered in Bartlesville, Okla., operates and owns 37 percent of the highly productive Ekofisk field, the first where oil was found and brought into production in the North Sea. Late last year, a decade after the first find there, the Edda platform started production.

Since it had room on board for just 85 people to live, it needed a floating hotel platform -- or "flotel" -- for the overflow during busy periods.North Sea oil workers spend days at a time at sea, working half of each day and relaxing or sleeping during the rest of it on the platform on a nearby flotel. Most large-production platforms are surrounded by satellite flotel platforms, many of which have been converted from drilling rigs.

In addition to the loss of life, the accident could have an adverse impact on oil production in Norway, where the government has been strict about the safety and environmental impact of drilling in the rough North Sea waters. The Norwegian Cabinet met in emergency session last night to receive reports of the accident and the rescue efforts.

Although North Sea oil already accounts for a large share of the national economic output of the country of just 4 million people, delays in production could result while the accident is investigated and other oil platforms checked or overhauled. A blowout that sent about 150,000 barrels of oil spilling out of an Ekofisk well in 1977 delayed drilling north of the 62nd Parallel, for which the Norwegian government only recently gave permission.