The United States and Vietnam agreed yesterday to "new levels of cooperation" and to accelerate efforts to resolve questions over the fates of servicemen missing in Southeast Asia.

Retired Army general John W. Vessey Jr., former chairman of the Joint Chiefs of Staff and special negotiator on this issue, met four hours yesterday with Vietnamese Foreign Minister Nguyen Co Thach and told reporters that Thach "agreed to the proposals I have made and I have agreed to the proposals he has made."

Those proposals call for increased cooperation from Vietnamese military officials and greater technical cooperation, as well as U.S. access to military archival and museum records to speed the search.

Thach said agreement also had been reached with Laos to have Laotian officials participate in three-part investigations to help locate remains hundreds of Americans believed missing along the Vietnamese-Laotian border.

Thach, the first senior Vietnamese official to visit Washington since the Vietnam War ended in 1975, insisted there were no U.S. prisoners of war being held by his government. Several organizations of servicemen's families say there have been numerous sightings of American POWs in Vietnam since the war, but the U.S. government has never confirmed those reports, according to a Pentagon spokesman.

Thach, who spent the rest of his one-day meeting with key lawmakers and the Senate Foreign Relations and House Foreign Affairs committees, said he was also optimistic that U.S.-Vietnamese relations could be normalized soon.

Relations have been virtually frozen over the missing servicemen questions and the decade-long war in Cambodia, which Vietnam invaded in 1978. The United States supports a three-party coalition fighting the Vietnamese-installed government.

But the permanent five members of the U.N. Security Council recently agreed to a plan for U.N.-supervised elections to end the war and the four factions also agreed in general to the plan. There is some prospect that the 20 nations involved in the Cambodian peace process may be able to sign a more detailed agreement in Paris before the end of this year.

U.S. officials have said that Vietnam has been cooperative so far in the peace process. For that reason, officials yesterday said there was what one called "an air of positive anticipation" surrounding Thach's visit.

Anne Mills Griffiths, head of the National League of Families of American Prisoners and Missing in Southeast Asia, who participated in the meeting with Thach and Vessey, said Thach had been cooperative and she was optimistic that the results would be positive.

Several officials said they had been encouraged by Thach in the past, however, only to find little or no cooperation once they arrived in Vietnam.

One senior official, however, said Hanoi's difficult economic situation, coupled with its likely loss of extensive Soviet aid would make cooperation with the United States much more in Vietnam's interest this time.

Some officials said that if a U.S. team scheduled to visit Vietnam finds the promised cooperation, it was likely that a permanent or semi-permanent search operation would be located in Vietnam soon.