U.S. helicopter forces, under increased attack from North Vietnamese gunners inside Laos, began yesterday to pull out of their command post at Khe Sanh near the Laotian border.

U.S. sources said the Khe Sanh base would be closed within three weeks as the helicopter units shifted operations eastward to their headquarters at Chu Lai and Phu Bai.

One U.S. officer said yesterday that North Vietnamese gunners in Laos may be hidden in old emplacements used by rebel troops in the 77-day seige of Khe Sanh in 1968. Built into the side of a mountain near the border, the emplacements cannot be damaged by air strikes.

Inside Laos, advancing North Vietnamese troops drove the remnants of South Vietnamese units toward the border. One South Vietnamese armored division fled into Vietnam under a heavy stream of fire.

Saigon headquarters reported yesterday that South Vietnamese forces remained at two bases in Laos- Hotel 1, two miles from the border, and Delta, seven miles inside Laos. Delta, however, was later abandoned after heavy attack, U.S. sources said.

Five thousand South Vietnamese troops- down from 22,000 at the height of the drive- still remain in Laos, Saigon headquarters said.

U.S. mechanized units continued to defend the South Vietnamese portion of Highway 9, guarding against a possible North Vietnamese thrust across that route.

U.S. officers said two airborne battalions struggled out of Laos on foot after being flown out of the combat area by helicopters, while helicopter gunships fired upon North Vietnamese positions less than a mile inside the Laotian border.

Saigon headquarters reported 13,672 North Vietnamese killed since the Laos invasion began on February 9. South Vietnamese casualties were listed as 1145 dead and 4199 wounded.

North Vietnamese forces claimed to have shot down six U.S. planes with SAM missiles during U.S. fighter-bomber raids on North Vietnam Sunday and Monday. These are the first planes reported downed by Soviet-built missiles since February 1968.