Israel's military headquarters in southern Lebanon was destroyed by an earth-shattering explosion yesterday that collapsed the eight-story building, trapped scores of Israelis and Palestinians inside and triggered fears in Washington of a new round of violence in Lebanon.

One Washington source said the blast had caused "a very large number of Israeli casualties."

While Israel refused official comment on the death toll during the day, Israeli dead were estimated to range as high as 50 to 85, according to one report reaching Washington. At least 15 Palestinian suspects being held there also perished, this report said.

The Associated Press reported from Tel Aviv that the military command issued a statement Thursday evening saying 13 Israelis had been killed and 25 wounded. But witnesses reported seeing as many as 60 bodies.

If the Israeli casualty figure turns out to be as high as the early estimates, it will be the greatest loss of Israeli lives in a single incident outside of combat since the founding of the nation in 1948.

There were an estimated 125 Israelis in the building in Tyre just north of the Israeli border when the blast reduced the structure to a pile of rubble. Rescuers were digging through the debris seeking victims, but the devastation was reported to be so great that it was estimated it may take them until Saturday to finish even a preliminary search.

Early reports that the explosion was caused by a car bomb were questioned by an informed Israeli source reached by phone.

He pointed out that buildings on either side were relatively undamaged, leading investigators to suspect that the cause of the blast was a large cache of explosives secretly -- and expertly -- planted in the headquarters structure over a period of days or even weeks. If a car bomb was involved, he speculated, it may have been used to detonate the stored explosives.

An emergency Cabinet meeting has been called for 10 a.m. today in Jerusalem to discuss what action Israel should take as as result of the explosion.

State Department sources said they feared Israel's response would be quick and violent, but they could only guess at the targets.

"You can figure they are going to blame the Palestinians and strike somewhere even though the guilty could just as well be Druze or Moslem Shiites or just about anyone," said one official who declined to be identified. "But whether it's in Tunis or Baghdad or northern Lebanon -- who knows -- you can figure they'll drop the other shoe."

The French news agency Agence France-Presse reported that its Beirut bureau had received a letter from an unknown organization called the Armed Struggle Group taking responsibility for the blast. "This operation will be followed by others to bring an end to the Israeli occupier and force it to evacuate our territory," said the letter. "We affirm that armed struggle is the ony way to eradicate the cancerous Israeli virus."

The Associated Press reported the following from Tyre:

Shortly after the 7:15 a.m. (12:15 a.m. EST) blast the Israeli spokesman in nearby Sidon, Lt. Col. Arnon Gonen, said: "There were deaths -- Israeli soldiers for sure." He would not give any numbers.

Israeli Defense Minister Ariel Sharon called the blast "a disaster whose cause and circumstances we don't know yet, and in which many people were hurt."

Israeli radio quoted "security forces" at the scene as saying that a suicidal attacker crashed a bomb-laden car into the building.

Reliable Israeli sources said the blast could have been caused accidentally, perhaps from ammunition in the building.

Israel television advanced other theories about the possible cause of the explosion--that the building was brought down by cooking gas tanks that exploded accidentally or by a small charge attached to one of its foundation pillars.

"I am not an expert in explosives," said Dr. Arieh Roth, an Army medic, "but if this was a terrorist act, it looks like very, very professional work."

Roth, one of the first on the scene after the explosion, said, "When we arrived, there were about 20 or 30 workers, Israelis and Arabs, on the ground. There was a very big disorder. A lot of women were crying. A few were really seriously wounded, but thank God none of them died under our care. After about 20 minutes medical forces of the Red Cross joined us, and we cooperated in a very nice way."

An Israeli soldier, quoted on Armed Forces Radio, said he had been asleep on the seventh floor and woke to find himself on the ground floor, with the screams of trapped and wounded soldiers ringing out.

Air Force helicopters swarmed overhead, evacuating victims, while Israeli troops and Lebanese Red Cross workers sought to dismantle the heap of rubble that was all that remained of the building.

The building stood on the coastal highway to Beirut, which was closed indefinitely by cranes, bulldozers and big jackhammers digging into the debris.