South Vietnamese troops invaded Laos early yesterday with massive U.S. air support, the South Vietnamese government announced last night.

In a communique issued in Washington and Saigon, President Nguyen Van Thieu said the thrust was aimed at cutting the Ho Chi Minh Trail and North Vietnam's supply and infiltration network in the Laotian panhandle.

The buildup for the sweep into Laos began 10 days ago with the movement of 20,000 South Vietnamese troops and 9,000 U.S. troops into South Vietnam's northwest corner along the Laotian border.

[The invasion climaxed a week of intense speculation that such a move would take place. Japan's Kyodo News Service reported last Tuesday that between 4000 and 5000 South Vietnamese troops had parachuted into Southern Laos on Monday. Kyodo reported yesterday that South Vietnamese troops supported by Americans had already driven 20 miles into Laos.]

Although the White House refused to comment on the invasion, the Associated Press reported that President Nixon is believed to have given the final order on the thrust.

At White House church services yesterday Nixon told a prisoner-of-war's wife that the United States would remain in Vietnam as long as American prisoners were held captive in the North, CBS News reported last night.

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U.S. officials in Saigon said last night that the South Vietnamese push is being given full U.S. combat support, including tactical fighter-bombers, helicopter gunships and troop-carrying helicopters. They said medical evacuation helicopters and logistic support will also be provided.

The U.S. command has said repeatedly that no U.S. ground troops would cross the frontier into Laos but unlimited airpower would be available to "protect withdrawing American GI's."

The South Vietnamese embassy announced the invasion minutes before Thieu issued his communique in Saigon. Thieu said his government has no designs on Laotian territory or internal politics and "will withdraw completely" at the end of the operation-code-named Lam Son 719.

Operations in the area of the Laotian border were shrouded in a six-day news blackout last week. Americans repaired and built bridges and airstrips, reoccupied the Khesanh fire base, scoured the countryside in search of nine North Vietnamese regiments, hit storage areas from the air and ground, and moved up to the Laotian border-all during official U.S. silence.