SAIGON (UPI) - The deputy commander of South Vietnamese troops defending the Saigon area was found shot to death Tuesday night following an argument with his superior over tactics. Military sources said he apparently committed suicide.

The sources said Maj. Gen. Nguyen Van Hieu was found with a bullet wound in his mouth at his III Corps office at the edge of Bien Hoa airbase, 14 miles northeast of Saigon.

It was not known whether Hieu's death was connected with the Tuesday morning bombing of the Presidential palace of Nguyen Van Thieu.

To the south and east of Saigon, Viet Cong and North Vietnamese forces continued heavy shelling and sapper attacks in what appeared to be efforts to close the circle around the jittery capital.

In Washington, U.S. Army Chief of Staff Frederick C. Weyand said South Vietnam cannot survive without additional military aid from the United States. He made the statement after reporting to the Senate Armed Services Committee on his recent trip to South Vietnam. If sufficient aid is received, the South Vietnamese will fight, he said.

The curfew resulting from the attack on the palace meanwhile slowed Operation Babylift and the air exodus of Americans, and the Viet Cong overran a district capital in the military district north of Saigon.

The pilot who bombed the palace took off from Bien Hoa for his mission and was still missing hours after the attack. Military sources said he probably defected to the Communists with his supersonic plane. Thieu escaped injury and later said the attack was not part of an attempted coup.

Hieu was deputy to Lt. Gen. Nguyen Van Toan, who commands troops in the 13 provinces surrounding Saigon. Military sources said the two generals got into a violent disagreement in an argument over military tactics, although the exact cause of the dispute between the two men could not be immediately learned.

On the battlefields, Communist forces moved closer to control of the II Corps area north of Saigon, overrunning one of the few remaining government positions at Thien Giao district capital, 115 miles northeast of Saigon late Tuesday, military sources said.

The sources said South Vietnamese defenders put up a tough battle and, backed by air strikes, killed 150 Communist forces. But human wave assaults overran the town at nightfall.

Civilians already have deserted the entire II Corps area where possible.

The last remaining government holdings in that area are around the province capitals of Phan Thiet and Phan Rang. But panic among government troops and Communist attacks on the edges of the cities made the situation so bad Tuesday that Western refugee workers were unable to land aircraft to help with civilian evacuation.

Bien Hoa airbase was hit by a rocket attack early Wednesday morning, but it was not immediately known how many rounds hit the strategic base.

About 300 mortar rounds hit a government post 45 miles east of Saigon, followed by a ground attack, spokesmen said. They listed nine Viet Cong killed and said the government side lost two men killed and two wounded.

Government and Communist officials said they had no idea where pilot Nguyen Thanh Trung, 26, a native of North Vietnam whose family lives in Communist held Da Nang had flown his F5 fighter after bombing Thieu's palace.

The curfew resulting from the bombing of Thieu's palace was lifted after six hours but it curtailed the human traffic on the daily airlift of orphans and civilian refugees from Saigon. At least one U.S. Air Force C141 Starlifter flight between Saigon and Clark Air Base in the Philippines was canceled and passenger traffic on two others totaled only 31 persons.