Excavation of a military aircraft crash site near Pakse in Laos was described yesterday as a major "breakthrough" in identifying the nearly 2,500 Americans missing in Vietnam, Laos and Cambodia.

"This is the first to be excavated and it's a big, big breakthrough," said Mary Currall, administrative assistant for the National League of Families of American Prisoners and Missing in South East Asia. "We know of hundreds more such sites and hope there will be many more excavations."

Most of the 576 Americans listed as missing in Laos are airmen, and the hope is that most of their remains would be found near crash sites such as the one at Pakse.

Ann Hart of Pensacola, Fla., a crew member's widow who visited the crash site two years ago as a representative of the National League of Families, said it would be a big step if it led to other excavations. She said she had steeled herself twice before to make the personal decisions that an excavation might bring on -- a year ago when it appeared that the excavation team would be admitted, and again last summer when President Reagan announced an agreement in principle with Laos.

"We'll have to decide whether to accept the identification decisions, what to do about burial, things like that," she said. "Of course we don't know how many will be identified, and if all are not, then some families will be left wondering."

She said a Pathet Lao cadre reportedly buried four or six of the crew members shortly after the crash, but she couldn't find the burial spot at the time of her visit from the information available to her.

"I assume those are the remains the team has found," she said.

The Pakse site has been the focus of negotiations between the United States and the communist government of Laos. Those negotiations were disrupted by statements of U.S. officials and actions of U.S. citizens, Currall said.

"We didn't get much cooperation from the U.S. government until President Reagan took office and made this a top priority," she said. "But then there was a hostile statement by Secretary of State George P. Shultz some time back that delayed matters.

"In 1983, he said that the United States knew that there were American POWs in Southeast Asia and that if necessary we would send in black helicopters in the night to get them. That angered the Laotian government and they broke off negotiations."

She said a POW search led by James (Bo) Gritz, a retired Army officer, also angered the Laotian government, which captured him, tried him, then released him.

In 1978, the Carter administration attempted to close the books in Southeast Asia by declaring that missing Americans were presumed dead, Currall said. Reagan made identification and return a top priority, however, reopening speculation that some Americans might be alive.

The Defense Department identified the 13 crewmen aboard the plane that crashed at Pakse as Capt. Harry Lagerwall of Carmel, N.Y.; Tech. Sgt. John Winningham of Grover City, Calif.; Maj. Francis A. Walsh Jr. of Westport, Conn.; Capt. Stanley Kroboth of Savannah, Ga.; Tech. Sgt. James Fuller of Cibilo, Tex.; Sgt. Robert T. Elliott of El Dorado, Ark.; Airman 1st Class Charles F. Fenter of Tucson; Capt. Thomas T. Hart III of Live Oak, Fla.; 2nd Lt. George D. MacDonald of Evanston, Ill.; 1st Lt. Delma Dickens of Omega, Ga.; Maj. Paul O. Meder of Jamaica, N.Y.; Airman 1st Class Rollie K. Reaid of Dora, Ala., and Capt. Robert L. Liles of Shreveport, La.