North Vietnamese gunners widened their shelling attacks on U.S. bases in South Vietnam's northern sector yesterday, forcing helicopters to abandon the forward combat base at Khe Sanh during periods of intensive fire.

As the North Vietnamese continued to pound the six-week-old South Vietnamese operation in Laos, battle-weary Saigon forces began preparing to end their invasion of the neighboring kingdom and pull back from forward bases in South Vietnam.

The Saigon command disclosed that between 2000 and 3000 South Vietnamese have been pulled out of Laos over the past two days.

North Vietnamese gunners shelled Khe Sanh in South Vietnam's northwest corner for the fifth straight day, Associated Press correspondent Holger Jensen reported from Quang Tri. About 500 shells have been lobbed into the base this week.

Since the airfield came under regular shelling attacks, most of the helicopters.used to support the Laos operation have been flown to Dong Ha, Quang Tri, and other rear bases.

In Saigon, South Vietnamese command spokesman Lt. Col. Tran An announced that "some South Vietnamese units which were heavily engaged by the enemy have been pulled back to South Vietnam. They have not been replaced. Just over a regiment was withdrawn."

At Ham Nghi, forward command post of the South Vietnamese 1st Corps and 1st Infantry Division near the Laotian border, Brig. Gen. Pham Van Phu, division commander, said the entire 1st Regiment of his outfit had been pulled out of operation and returned to Vietnam.

Soldiers at Ham Nghi were seen loading more than 100 trucks with desks, beds and other supplies. Some bunkers were emptied and some tents taken down.