The United States and Vietnam have agreed informally to hold a new round of talks about establishing relations during the United Nations General Assembly session starting late next month, U.S. officials said yesterday.

Officials said both sides agreed in diplomatic contacts that the General Assembly session would be a good time and place for such a meeting, though dates have not been set for it.

The meeting would be the first since last December. There have been repeated public and private signals that Hanoi has dropped its demand for $3 billion in war reparations as a precondition for diplomatic relations.

In Hanoi, meanwhile, leader of a U.S. congressional delegaton said yesterday that his group had reaffirmed its earlier conclusion that 3209 American servicemen still listed as missing in action during the Indochina war are all dead.

"I think it's the general feeling of this group that there are no Americans still alive in Southeast Asia," said Rep. G.V. (Sonny) Montgomery (D-Miss.).

Vietnamese officials told the eight congressmen Monday that farmers had located the bodies of 11 pilots who crashed in remote areas and would return the remains to the American lawmakers.

After a series of meetings with top Vietnamese officials, however, Montgomery said it was time for the families of the remaining 329 missing men to give up hope that they are still alive. Six of eight lawmakers concurred. In December 1976, the committee issued a report that came to the same conclusion.

"I believe we should close the chapter on hope," Tony Won Pat, Guam's delegate to Congress and a member of the House Armed Services Committee, said.

The congressmen, who met with Vietnamese Premier Phan Van Dong and other officials, said Vietnam appeared almost desperate to normalize relations with the United States, its former enemy.

One major reason, the Vietnamese said, was that China had supplied troops and artillery to Cambodia, which is fighting a bitter border war with Vietnam.

Vietnamese spokesman Ngo Diem told U.S. reporters accompanying the delegation that Peking sent "advisers and cadre" as well as heavy 130mm artillery to help the Cambodians.

Asked whether Chinese troops were taking a direct part in the fighting, Diem said, "Yes."

The only two congressmen still willing to hold out hope for finding missing American servicemen were Rep. Sam Hall (D-Tex.) and Rep. Henson Moore (R-La.).

"I just have a gut feeling that one of these MIAs will show up someday," Hall said. "Sonny, I wish I shared your view but I don't."

"I wish I could share your view. I don't," Montgomery said.

Moore leaned forward from his chair. "I'm not willing to say all are dead. The probability any are alive is low, very low. But there is a chance."