While ground-fighting continued in Cambodia, 128 U. S. planes bombed North Vietnam in the first massive bombing raid since November 1968.

According to unidentified Administration sources, the targets of the attack were Communist supply routes into Laos.

Official White House spokesmen explained the raid as an action against anti-aircraft installations to protect unarmed U. S. reconnaissance flights over North Vietnam.

In the Fishhook area of Cambodia, U. S. Air Force bombers yesterday destroyed part of Mimot, a rubber plantation town, after North Vietnamese troops reportedly fired on U. S. Army helicopters.

The U. S. 11th Armored Cavalry Regiment yesterday burned at least five 30-40-hut villages in another part of the Fishhook. Officers explained that the villages were burned because they could be of use to the Vietcong and the North Vietnamese.

In central Cambodia. Communist troops won an important river crossing 37 miles southeast of Phnom Penh, the national capital.

In keeping with the "get tough" line of President Nixon's Thursday-night speech. Secretary of Defense Melvin R. Laird warned Saturday that he would support a resumption of the bombing of North Vietnam if Hanoi responds to the recent American tactics by sending troops across the demilitarized zone.

Furthermore. Laird said, in the future, "Americans who remain in South Vietnam will not be sitting ducks for an enemy that can hide in sanctuary areas." Three air raids, as well as two ground campaigns, have already been launched against Cambodian sanctuary areas. Laird added.

In a televised interview yesterday, Vice-President Agnew said that the U. S. air strikes were in accordance with the traditional American policy of protecting unarmed reconnaissance planes from anti-aircraft fire.

U. S. and South Vietnamese ground troops have not yet found the major Communist base complex, reportedly located in the Fishhook region of southeastern Cambodia.

Americans are searching for an underground, multi-story complex which they believe is the central Vietcong command headquarters and is situated in this area.

A CBS poll reported last night after telephone interviews with 1022 people that Americans support President Nixon's decision to send U. S. troops into Cambodia by a margin of almost two to one.