A U.S. delegation concluded two days of "frank" negotiations in Hanoi on the issue of missing American servicemen today and reported that the Vietnamese had accepted an invitation to visit the Defense Department's identification facilities in Hawaii.

The five-member U.S. team led by Deputy Assistant Secretary of Defense Richard A. Armitage, the highest ranking Pentagon official to visit Vietnam since the Communists took over in the south in 1975, arrived in Bangkok today stressing that they had discussed only the issue of the 2,500 American servicemen still listed as missing in action in Indochina.

"We did not discuss any other issues," Armitage told a brief news conference here. However, he would not say whether the Vietnamese had raised other matters of greater concern to them, notably the questions of diplomatic relations with the United States and American efforts to block international aid to Hanoi because of its three-year-old occupation of neighboring Cambodia.

The participation in the U.S. delegation of a State Department official had stirred speculation that other issues might be taken up.

"We made a very conscious effort to impress on the Vietnamese our determination to pursue this difficult question," Armitage said of the efforts to account for prisoners of war and missing servicemen. "We felt we developed a spirit of cooperation on this issue."

However, it was clear that the talks produced nothing concrete to advance a full accounting of the missing Americans.

In reply to a question, Armitage said the fact of the U.S. team's visit to Hanoi was "a sign of progress" on the still emotional question, as was the Vietnamese acceptance of the U.S. invitation to visit Hawaii for further talks.

In a prepared statement, Armitage announced that "the Vietnamese Office for Seeking Missing Persons will send a delegation to visit the Joint Casualty Resolution Center and the Central Identification Laboratory in Honolulu."

No date for the visit or further meetings was announced. According to the statement, the two sides agreed only that "the specialists of both sides will continue to meet when necessary."

Armitage said the Defense Department had received more than 300 "live-sighting reports" about American servicemen allegedly seen in Indochina after the end of the Vietnam War.

"We have to assume the reports may be true to prosecute our investigations," Armitage said. He added that 170 of the reported sightings had been investigated and "no credible evidence" had been found to substantiate them.

According to reporters allowed by the Vietnamese authorities to cover the U.S. team's trip, Hanoi made it clear that further contacts on the issue would be abandoned if authorities felt the Reagan administration was using the MIA issue politically against Vietnam.

The delegation, which included Air Force Lt. Col. John Fer, who spent six years as a prisoner of war in Hanoi, arrived in the Vietnamese capital yesterday to a chilly reception.

Vietnamese Foreign Minister Nguyen Co Thach told U.S. reporters who preceded the team to Hanoi that Washington must change its attitude if it wants further Vietnamese cooperation.

"They use the MIA problem as a political means against Vietnam, and so we told them no more cooperation," Thach said, according to United Press International.

"If we continue to search for the missing, we do it as a humanitarian act for the American people--as an act of friendship for the American people," the agency quoted him as saying.

"We have done our best," Thach continued. "We did it all for free. They did not pay anything."

However, the foreign minister said Vietnam would not permit Washington to send its own search teams to Vietnam, UPI reported.