The Pentagon is counting heavily on technology, including radars that pinpoint the origin of incoming mortars, to keep Marine casualties at a minimum in Beirut, a high defense official said yesterday.

The idea is to fire back so heavily and effectively that such attacks will be deterred. The official said a warning round is fired first, followed by a barrage if the shelling continues.

The warning is a 155-mm round that lights up like a flare over the attacker's position. The follow-up can be Marine mortars, their 155-mm artillery shells, Navy 5-inch guns offshore or rockets or machine-gun fire from Cobra helicopters based on an amphibious assault ship.

The official said the illumination round is often enough to halt hostile firing. The origin of the fire has been pinpointed again and again, he continued, by a special Army radar unit working with the Marines dug in around the Beirut airport.

The radar, called Fast Tab, is a marked improvement over those used to pinpoint enemy mortar positions in the past, officials said. Fast Tab has been so helpful in telling gunners where to direct warning rounds or lethal fire that hostile mortars often are silenced in minutes, one official said.

Even with Fast Tab, however, mortars can land and explode before Marines have enough warning to take cover, as happened when two Marines were killed in Beirut on Monday. If an enemy mortar tube is very close and conditions are quiet, a Marine might hear the shell's clunk as it is dropped down the tube and then dive for cover. Usually a mortar explosion is preceded only by a last-second flutter of sound.

Defense Secretary Caspar W. Weinberger said yesterday that the United States responsibly cannot "throw up our hands" in the wake of the Marine casualties.

"What is essential," he said in an interview with Newhouse News Service, is "to assist the Lebanese armed forces, the representatives of the official, legally seated government, to gain order and control of the country."

He added that "there is no disposition to change the makeup or the numbers of the multinational force that I know of, not among the French, Italians or British or among ourselves." He cautioned, however, that "everything can change within half a minute, half an hour."