A senior U.N. relief official has visited Phnom Pehn, where he apparently discussed arrangements to allow large numbers of Cambodians now in Thai border camps to voluntarily return home as food shortages in their country are eased.

Zia Rizvi, the U.N. High Commissioner for Refugees' Bangkok-based special coordinator, left Phnom Penh Feb. 4 after spending at least three days there. Reached by telephone in Geneva later, Rizvi confirmed he had made the trip but he would not discuss details of his talks.

A U.N. spokesman in Geneva, however, called the trip very successful.

About 400,000 Cambodian refugees are living in the Thai-Cambodian border region, 150,000 of them in U.N.-sponsored camps and the rest in primitive border settlement controlled by guerrillas loyal to either the ousted Khmer Rouge regime or the rightwing Khmer Serei.

Hostilities between some of the refugees and the Heng Samrin regime in Cambodia have given rise to fears here of retaliatory military action inside Thailand by Vietnam, whose forces support Heng Samrin.

In Phnom Penh, Rizvi met with ranking members of the Heng Samrin government, including Foreign Minister Hun Sem, sources said. He also conferred with Western relief workers stationed in the Cambodian capital. During the trip he passed through Hanoi, where he talked with Vietnamese officials.

Rizvi, a Pakistani national, was the first U.N. refugee official to confer directly with the year-old Heng Samrin regime. Sources in Bangkok reported he raised the question of voluntary repatriation.

Details of his proposals were not available. Many aid workers feel that food shortages -- not oppression by the Vietnamese -- were the prime reason most refugees moved to the border and large numbers will want to go home if sufficient supplies of food are guaranteed.

Some aid specialists wonder whether the presence of the border camps and the handouts of rice there will work against efforts to get a full crop of rice planted in Cambodia this May and June.

U.N. policy is that repatriation -- on a strictly voluntary basis -- is the best solution. The sheer numbers involved in the Cambodian exodus, which began late last summer, make resettlement in other countries like the United States impractical, refugee workers say.

Until last week, Thai Army officers, citing security problems, stopped food distribution at the frontier village of Nong Chan. Foreign agencies were passing out rations for an additional 20,000 Cambodians to people who showed up there on bicycles and oxcarts and on foot to carry the food back into Cambodia.

While refugees who leave Laos and Vietnam generally have no intention of coming back, Cambodia produces significant numbers of short-term refugees.

On jungle trails close to the border, there is sometimes an unbroken stream of people walking into and out of Thailand. Some come to the border camps to have a look, then return to their villages to get their families. Others stay only long enough to pick up food, then head back into the Cambodian interior.

In a few cases repatriation might already be happening. A family might simply pack up its things and walk home. Vietnamese and Cambodian soldiers might confiscate rice and black-market goods they acquired in Thailand, but the troops are not making a serious effort to stop those returning.

Observers here say the Phnom Penh authorities do not object in principle to repatriation from the Khmer Serei camps. President Heng Samrin said recently that he hoped all Cambodians would return to their homes. Asked what type of people were in the border camps; he said a few were Khmer Rouge, a few Khner Serei but the majority were ordinary people with no connection to the resistance groups.

Observers think repatriation from Khmer Rouge camps would be another question, however. Relatively few civilians live there and most of those that remain are closely associated with the Khmer Rouge's insurgency against the Vietnamese Army. There is no known commerce between the Heng Samrin and Klmer Rouge zones.

The more than 32,000 people in the U.N.-operated camp at Sa Kaew, where Khmer Rouge cadre still enforce their ascetic life style, would be "trained" in the eyes of the Heng Samrin authorities, one Western diplomat predicted.

Even if repatriation is accepted in principle, agreeing on operational details might prove difficult. The repatriation accord Thailand signed with Laos last year (there are about 140,000 Laotians in Thai camps) allows Laotian officials to reject individuals. Cambodian officials might want the same right.

One diplomat here expressed skepticism that refugees would submit to selection interviews. Bringing Heng Samrin's officials into Thailand could also present problems, since Thailand has not recognized their government.

Moreover, the Klmer Rouge and Khmer Serei might use force to block mass repatriation. The resistance groups look on the civilians as their base of support and source of international legitimacy.

Some Cambodians will refuse to return home. Many of the 110,000 at the U.N.-supported camp at Khao i Dang have written to foreign embassies in Bangkok requesting resettlement abroad.

The first 82 Cambodians in Khao i Dang to be accepted for foreign resettlement were scheduled to leave the camp this week, 69 for the United States.