Thieu Warns of Northern Invasion

South Vietnamese President Nguyen Van Thieu warned yesterday that South Vietnam "may be forced" to invade North Vietnam. He also defended incursions into Laos and Cambodia as "entirely for our protection."

An attack on North Vietnam could occur if the North Vietnamese "put us in a position which forces us to think about that option to legitimately defend ourselves," Thieu said.

Great Awakening

Thieu said he hoped the North Vietnamese "would soon awaken to the reality and not put us in a situation which forces us to attack them right in their own territory."

Sovere fighting continued yesterday in southern Laos and eight U. S. helicopters flying support for South Vietnamese troops were shot down, the U. S. Command reported. It was the largest loss of helicopters in a single day since the Laotian invasion began on February 8.

The U. S. Command also reported that North Vietnamese surface-to-air missiles were directed against American aircraft over South Vietnamese territory for the first time yesterday.

About 2000 Vietnamese marines have been moved into Laos in the past few days, raising the number of South Vietnamese troops there to 18,000.