With almost no publicity, large numbers of Cambodian refugees have returned home in the past year from camps on the Thai border and in Vietnam and Laos.

Some have made the journey voluntarily, attracted by reports of improving food supplies and security. Others have moved only with the prodding of soldiers.

The returnees number about 300,000 according to the Heng Samrin government in Phnom Penh, including 35,000 brought back, apparently forcibly, by Vietnamese troops who briefly occupied two refugee settlements on the Thai border in June.

Representatives of the U.N. High Commissioner for Refugees, meanwhile, have visited Phnom Penh repeatedly this year to discuss assistance to civilians who have returned to their native villages, aid sources said. No agreement has yet been reached, however.

Besides obtaining assistance, the new Heng Samrin regime is believed to be anxious to open links with the U.N. refugee organization as a way of enhancing Phnom Penh's legitimacy. The fallen Pol Pot government still occupies the Cambodian seat in the General Assembly.

Furthermore, publicity about a mass return of Cambodians from abroad would help to demonstrate to the outside world that it has full control of the country and has regained the confidence of those citizens who fled.

This homeward traffic has not, however, created optimism here that the refugee problem eventually will fade away. Almost 200,000 more Cambodians remain in camps well inside Thailand and Vietnam, most of them apparently determined to hold out for resettlement in third countries, in particular the United States.

For months, refugee workers have been aware of a steady flow of Cambodians back into their country. However, Phnom Penh's figure of 300,000, quoted in talks with aid officials, is questioned by some Western diplomats here.

It may have been inflated to secure more U.N. aid, it is argued, or to create the impression of a massive vote of confidence for the diplomatically isolated government, installed by a Vietnamese invasion force 20 months ago.

By Phnom Penh's count, about 160,000 of the refugees came from the Thai border. This apparently refers to former residents of camps located in disputed territory north of the Thai town of Aranyaprathet and controlled by right-wing Khmer Serei guerrillas.

These unofficial settlements were founded last year as thousands of Cambodians brought whatever gold they had to the Thai border to trade for food and other goods. Their population probably peaked early this year at something under a quarter million.

But with gun battles between rival Cambodian guerrilla groups erupting frequently inside the camps, and with food slowly improving in western Cambodia, many families seem to have made the decision to pack up and walk back down the jungle trails toward home.

In many vilages, the Heng Samrin authorities created "reception committees" to screen out agents of the deposed Khmer Rouge government of Pol Pot or the Khmer Serei. However, most people were reported to have passed the test with little problem.

In June, Vietnamese forces occupied two Khmer Serei-controlled border camps as part of their brief incursion into Thailand. Officials in Phnom Penh have said that 35,000 people were led away from the camps and taken to a site south of the district town of Sisophon in western Cambodia.

A special screening group directed by Cabinet ministers cleared most of the people within a few days to return to their home villages. However, about 100 men were still detained weeks later, apparently on suspicion of involvement with the Khmer Serei.

This was not the first time soldiers had forcefully repatriated Cambodian refugees, however. One year earlier, the Thai Army bused more than 40,000 of them to an isolated border crossing and pushed them across at gunpoint.

The return from the border camps has proceeded independently at Thai government programs to send back volunteers among the 150,000 Cambodians living in U.N.-financed "holding centers" well inside Thailand.

About 9,000 people went back to Cambodia before the U.N. program ended in June. Most were Khmer Rouge loyalists. Reentering Cambodia at points controlled by Khmer Rouge guerrillas, many doubtless took up arms against the Vietnamese.

Refugee workers see little chance that the majority of the 150,000 people living in U.N. holding centers would ever choose to return to Cambodia, if given a free choice. Highly educated or with experience in government or business, many hope to reach the United States or Europe.

But with other countries having quotas for only a fraction of that number, officials in Bangkok fear Thailand will be saddled with camps full of refugees indefinitely, unless they are forcibly sent home.

Last year, Vietnam faced a similar problem. In the years before the Khmer Rouge were overthrown, close to 150,000 Cambodians crossed into Vietnam and took shelter in temple compounds, provincial towns and special camps in the southern provinces.

Some were genuine refugees, fleeing government purges and social regimentation. But as many as half were Cambodian villagers rounded up by Vietnamese Army units making limited forays into Cambodia before the all-out invasion of December 1978.

Hanoi seems to have decided it needed a larger pool of Cambodians from which to recruit a civil service an Amy fo the Heng Samin government it later installed. Then, as Vietnamese forces routed the Khmer Rouge in province after province, the new Cambodian administrators followed close behind to begin their duties.

However, about 150,000 Cambodians remained behind in Vietnam. Then between June and October last year, 115,000 of them recrossed the border to settle in Svay Rieng or Prey Veng provinces, where most had originated.

It appears these people did not have the choice of staying in Vietnam. However, the remaining 35,000 Cambodians, most of them ethnic Chinese, did not have the choice to leave. The old commercial class they belong to has not been given a role in the new Cambodia, Hanoi believes. It has proposed that third countries resettle these people directly from Vietnam.

Meanwhile, another 20,000 Cambodians have also left southern Laos, where they fled during Pol Pot's years in power.