Lt. Gen Do Cao Tri, South Vietnam's top field commander, was killed Tuesday in a helicopter crash en route to inspect more than 20,000 Saigon troops on a major offensive in eastern Cambodia.

Field reports said the 42-year-old general's helicopter crashed after developing mechanical trouble shortly after take-off from his forward headquarters at Tay Ninh, on the Vietnamese side of the border.

Tri was directing a thrust by 20,000 Saigon troops into Cambodia. He also led the major incursions into Cambodia last May.

On the field, South Vietnamese troops were stalled under heavy pressure from North Vietnamese and insurgent troops. Sources said the Laos drive was behind schedule and that one reason for the halt was heavy concentrations of mines along the Ho Chi Minh trail.

The Laos and Cambodia drives, involving more than 40,000 South Vietnamese troops, are aimed at cleaning out North Vietnamese base camps and supply points along the Vietnamese border.

Tri, an aggressive commander who was often on the front lines, often dared danger and once said in an interview: "Maybe ten times everyone around me was killed or wounded. I have never been touched by an enemy bullet."

He is the highest ranking general, American or South Vietnamese, killed in the Indochina war.

In Washington, Defense Department officials acknowledged the South Vietnamese suffered a setback in Laos over the weekend, but said that was "no reason to write off the whole operation." The State Department said the objective of disrupting the flow of North Vietnamese supplies "is being carried out."