By REUTERS

BEIRUT, Lebanon, Sunday, Oct. 23 -- 

Forty-three United States Marines and 15 French soldiers were killed today in dawn explosions that razed two buildings used by the multinational peace force in Beirut, the state-run Beirut radio said.

Three civilians also died in the rubble of the Marines' battalion landing team's headquarters building, effectively their main headquarters near the Beirut International Airport.

First reports said the blasts were caused by car bombs set off by remote control, though the extent of the damage suggested the explosives were placed inside the buildings.

(United Press International quoted a Marine spokesman, Maj. Robert Jordan, as saying, "A truck full of explosives crashed through the gates and into the Marine battalion headquarters building.")

Bodies of 15 Frenchmen Found

A spokesman for the Lebanese Civilian Defense said rescue workers had recovered the bodies of all 15 French soldiers from a four-story building used by their contingent to the multinational force.

(U. P. I. reported that one French officer said as many as 100 French soldiers may have been "wiped out.")

Randall Palmer, a Reuters correspondent, who got to within sight of the building used by the United States force, said the five- or six-story structure had collapsed into one story. Slabs of concrete were folded down over the edges.

Troops Seal Off Area

Many marines slept in the building, and it was not immediately clear whether the casualty toll could still rise.

United States troops sealed off the area, but Mr. Palmer could see bulldozers at work clearing the rubble.

The ceiling of a nearby building used by United Nations troops collapsed from the force of the blast, but there were no casualties, military sources said.

U.S. Confirms Blasts

As of 3:15 this morning, Brian Carlson, a State Department spokesman, confirmed that "there were two explosions, one of which affected a building near the marines, or perhaps was the Marine headquarters." He said he did not know how many Americans had been killed.

A Defense Department spokesman, Lieut. Col. Thomas Jones, also confirmed the two explosions, one in the Marine area and one in the French area, but said the department did not yet know the extent of the damage or casualties.

A White House spokesman, Sheila Dixon, said that President Reagan was awakened at 2:27 A.M. and told of the bombings by his national security adviser, Robert C. McFarlane. The spokesman said the White House was monitoring the situation.