SAIGON--Nearly 100 U.S. B-52 bombers and a naval task force of more than a dozen cruisers and destroyers blasted North Vietnamese positions on both sides of the demilitarized zone Monday in some of the heaviest bombardments of the war.

The raids were in support of a 20,000 man South Vietnamese counter-offensive on the northern front. They had the two-fold objective of destroying North Vietnamese troops and supplies already on the battleground and cutting off reinforcements and war materials reportedly moving south.

The Saigon command reported light fighting on the edges of Quang Tri, the provincial capital which fell to the North Vietnamese May 1. It said South Vietnamese paratroopers driving from the east had inched their way to within 200 to 300 yards of the Citadel, the 19th century fortress in the heart of the city.

The government has maintained that its strategy is to destroy North Vietnamese forces around the city before attempting an all-out assault against the large and heavily-armed forces reported entrenched in bunkers in the city.