From Wire Dispatches

Amid speculation that the Allied invasion of Laos may be turning into a second Dien Bien Phu, Nixon administration spokesmen announced yesterday that American combat troops will be sent into Laos should it prove necessary to protect "search-and-rescue teams" trying to save downed American airmen.

Pentagon spokesman Jerry W. Friedham said yesterday that the use of infantrymen to guard rescue operations represents "absolutely no change" in administration policy.

'Protective Encirclement'

White House officials called the practice "protective encirclement," and said that the presence of American troops would not constitute any violation of legislative restrictions on the use of American ground troops and advisors in Laos.

The Cooper-Church amendment passed by Congress last year bars the Nixon administration from introducing combat troops or advisors into Cambodia and Laos, although it does make provision for the President to act in order to save American lives.

Pentagon spokesmen reported that the North Vietnamese moved several thousand fresh troops into the Laotian panhandle during the past week. Large numbers of North Vietnamese troops and tanks were reported yesterday moving toward the main South Vietnamese force in the Laotian panhandle.

Hill 31, a South Vietnamese paratroops base which has been hotly contested over the last five days, fell to North Vietnamese forces yesterday despite constant American air bombardment.

Some U.S. officers in the field have called the Laos fighting the heaviest of the Indochina war. In addition, South Vietnamese losses are termed the heaviest since the Tet offensive of 1968.

It was also reported late last night that North and South Vietnamese forces in Cambodia have engaged in their heaviest clashes since last May, when an Allied force led by American ground troops invaded that country.

The latest Allied position under siege was another paratroop base about six miles southeast of Hill 31, which is eight miles inside Laos.

In the Senate, acting majority leader Robert C. Byrd said he thinks "there would be a great deal" of Senate backing for a proposal to bar U.S. air support for any South Vietnamese invasion of North Vietnam.

"I don't think the American people would support such an invasion," Byrd told reporters yesterday. He added that he might support the proposal by Sen. Walter F. Mondale (D-Minn.), and 18 other senators, barring an American invasion of North Vietnam.

South Vietnamese President Nguyen Van Thieu was quoted Thursday as saying that an invasion of the North was only a matter of time. Thieu's press secretary later said that the official South Vietnamese press agency had misquoted Thieu.