WASHINGTON--A disabled Soviet nuclear submarine sank east of Bermuda before dawn yesteday, the Pentagon announced. Sources said all the crew were "presumed" to have escaped.

A Pentagon spokesman, Col. Marvin Braman, said survivors were seen being picked up by Soviet merchant ships, one of which had been towing the Yankee-class sub.

Two officials, who demanded anonymity, said that fewer than 24 men were believed left on board for the towing operation and that life rafts were seen moving toward a merchant ship before the sub finally slipped beneath the surface three days after it was wracked by fire and an explosion.

"It is presumed that everybody got off, but we don't know for sure," said one source. "They certainly had enough time to get off, because this boat was clearly slipping lower in the water more than three hours before it went under."

The sub sank in 18,000 feet of water 1040 nautical miles east of Cape Hatteras, N.C., at 4 a.m., EDT, Braman said, almost four hours after the towing ceased and the submarine was seen to be taking on water.

There was no immediate word on casualties from the sinking. One official said earlier, however, that many of the sub's crew had been evacuated to nearby Soviet merchant ships. The sub normally carries about 120 people.

In Moscow, a spokesman for the Foreign Ministry said he had "no information at all about those reports" of the sinking.

Braman said a Navy P3-C patrol plane reported the sinking more than 500 miles north of Bermuda and 80 miles from the point where Friday's fire and explosion killed three men.

Braman noted that the sub had encountered rough seas as it was being towed through the Atlantic.

He added that "a reasonably prudent individual would have to assume that [with] the kind of damage that was shown in the photos, ...you're going to be taking on some water."

One source had noted that the submarine, designed to carry 16 nuclear-tipped missiles, had apparently sustained damage to its hull and possibly one missile tube hatch cover.

Beyond that, the Pentagon had no immediate explanation of why the submarine sank.

The sources also declined to say whether the United States had been able to monitor the status of the sub's two nuclear reactors as the vessel sank.

But a retired U.S. Navy officer, who commanded the nuclear-powered submarine Polaris, said officials had studied such a "worstcase scenario" and concluded there would be "no danger from this."

James Bush, a retired captain and now associate director of the Center for Defense Information, a private Pentagon watchdog group, said any radioactivity from the nuclear reactors would be diluted by the sea.