Four U.S. veterans of the Vietnam war today ended an emotional six-day return visit to that country and said the Hanoi government had agreed to help private American citizens trace missing servicemen and investigate the effects of the defoliant Agent Orange.

During a brief stopover here on their way back to the United States, the veterans told reporters that the agreements resulted from a meeting with Vietnamese Foreign Minister Nguyen Co Thach. They said Hanoi also agreed to receive more delegations of veterans through their organization, Vietnam Veterans of America, and to develop a "cultural exchange" program with the group.

The veterans, the first such group to visit postwar Vietnam, declined to answer questions but made brief statements to reporters at Bangkok's airport after flying in from Ho Chi Minh City, formerly Saigon.

They said they had met several times with Vietnam's committee on missing servicemen, but the veterans made no mention of any new information on the sensitive subject.

According to U.S. officials, about 2,500 servicemen are listed as missing in action in Vietnam, Laos and Cambodia. Of that number, around 50 are known to have been captured alive, officials said.

The U.S. government has had a number of official contacts with Vietnamese authorities in efforts to resolve the issue. At the last meeting in Hanoi in May, the Vietnamese identified three dead airmen who had been listed as missing in action. Their remains were later flown to the United States.

U.S. Embassy officials here said the American government encouraged the efforts of private groups to investigate. But the officials said Hanoi was evidently more interested in publicizing inquiries into the effects of Agent Orange, partly to deflect U.S. accusations that the Vietnamese or their allies have been using chemical weapons against opponents in Cambodia and Laos.

In the veterans' statement today, Robert O. Muller, the group's leader, said Foreign Minister Thach "pledged to continue working with us in a cooperative effort to help resolve the question of the American servicemen that are still listed as missing in action."

Muller, 36, a former marine who was paralyzed from the waist down by a bullet wound, said the veterans and the Vietnamese agreed "in principle" that U.S. doctors and scientists would be allowed to visit heavily defoliated areas of Vietnam to carry out investigations into the effects of Agent Orange.

Muller said the group had conferred "a couple of times" with a Vietnamese committee investigating "the consequences of toxic chemicals" and had met Hanoi's leading expert on the subject.

The veterans also toured the Little Flowers Hospital in Ho Chi Minh City, where they saw "grossly deformed" children, Muller said. He said some were deformed because of congenital birth defects and others were possibly victims of "toxic chemical exposure." Vietnamese authorities say some of the abnormalities were caused when expectant mothers became exposed to Agent Orange.

Muller said the group visited an orphanage where children of American servicemen are housed, met a group of disabled North Vietnamese soldiers, visited the war museum in Hanoi and toured the mausoleum and former home of the late leader Ho Chi Minh. Muller did not mention an incident, reported by a New York Times correspondent accompanying the group, in which the veterans reluctantly agreed to lay a wreath on Ho's tomb.

Muller said that during its four days in Hanoi, the group was able to "walk about freely," enter shops and restaurants and "visit many people on the streets." He said life in the capital "can only be called spartan" but that the people received the group warmly.

Two other veterans, former infantryman Tom Bird and ex-Navy seaman John Terzano, also said they were impressed by the friendly reception.

The fourth member of the group, former Air Force sergeant Michael Harbert, said the experience "probably changed our lives." A veteran of 47 bombing missions over North Vietnam, Harbert added, "The Vietnam war caused a great deal of pain in the United States, and we as veterans felt that pain." This "gave us all the opportunity to understand that the war is in fact over."