U.S. Military sources reported yesterday that rocket and mortar shelling of Khe Sanh, a South Vietnamese supply point and flight center for operations into Laos, has been the heaviest since 1698.

Khe Sanh had been abandoned since American Marines manned the base in 1968 during a 77-day siege. When the Laotian campaign began in early February the site was reopened as an important base. Three helicopters were damaged during the attack yesterday but were able to fly out safely to Quang Tri.

After several days of relative quiet, fighting has began again near Sepone on the Ho Chi Minh Trail. U.S. sources reported heavy North Vietnamese artillery and tank attacks at Lolo, a fire support base nine miles southeast of Sepone. One American helicopter was reported shot down, and several others hit. Helicopter crewmen said that they were forced by heavy ground fire to drop supplies while hovering five or six feet above the base.

The U.S. Command yesterday claimed that the number of American troops in South Vietnam has reached 317,300. the lowest point since October 1966. U.S. headquarters has explained that the drop is a direct result of Nixon's plans for troop withdrawals in South Vietnam.