U.N. refugee officials are looking into the fate of more than 300 Laotians who were forced by Thai authorities to return to their country three months ago after fleeing the Communist government there.

A team of officials representing the U.N. High Commissioner for Refugees (UNHCR) is scheduled to visit the native villages of the Laotians this week to check whether they returned safely. Concerns have been expressed here that some of the refugees might have faced reprisals by the Vietnamese-backed Pathet Lao government in Vientiane.

The UNHCR office here formally protested to Thai authorities what it called the "push-back" of 335 Laotians in four incidents in early June and "expressed its objections on a number of subsequent occasions," a UNHCR spokesman said. "This is a matter of continuing discussions with the royal Thai government," he added.

The forcible repatriation came amid indications that Thailand is taking a tougher line against Indochinese refugees, who have continued to trickle into the country across two borders and by sea more than seven years after the Communist takeovers of Vietnam, Laos and Cambodia.

The stricter attitude appears to stem from Thai concerns that although the rate of refugee arrivals has declined, Western countries are accepting fewer of those here for resettlement than last year. Thai authorities fear that as the Western nations tighten their refugee policies, Thailand will be burdened by an increasing number of refugees nobody wants.

The fear lies behind the Thai policy of "humane deterrence" to discourage would-be refugees from leaving their homelands in the first place by putting them in spartan camps when they arrive and denying them eligibility for resettlement.

So far the policy seems to be working. Some 200 to 300 refugees a month arrive in Thai camps from Laos now compared to 2,000 to 3,000 a month at the peak of the exodus in 1979-80, refugee officials estimate.

The arrivals of Vietnamese "boat people" and Cambodian refugees who manage to sneak across the officially closed border are also down sharply, the sources said.

Across the region, "humane deterrence" has combined with a tighter U.S. policy, a reported Vietnamese government crackdown and a shortage of boats to halve the number of boat people arriving on Southeast Asian shores in recent months compared to last year. According to UNHCR figures, about 15,000 Vietnamese boat people arrived in May, June and July this year.

However, according to Squadron Leader Prasong Soonsiri, secretary general of the Thai National Security Council, third countries took 19,500 refugees from Thailand for resettlement during the first six months of the year compared to 48,000 in the same period last year.

"This tendency confirms the . . . anxiety that eventually Thailand will be burdened alone," Prasong told a recent refugee conference. "Thailand is determined not to allow even one displaced person left on her soil," he said in a tough speech.

He warned that "if all countries and international organizations fail to honor their commitments and Thailand has to bear these grave burdens alone, I deem it necessary for Thailand to take a new step in order that our interests and security can properly be preserved."

Prasong did not elaborate, but Bangkok newspapers interpreted the statement as a threat of drastic measures that might include repatriation of Indochinese refugees. In June 1979 Thai military authorities forcibly repatriated more than 40,000 Cambodian refugees during fighting between Vietnamese invasion troops and Khmer Rouge guerrillas.

According to U.S. figures, the number of Indochinese refugees in Thailand peaked at 305,000 in February 1980 and now stands at about 177,000. "But now the curve has flattened out," one refugee official said.

Worried about the prospect of more forcible repatriations, the U.S. government also has made its concerns known to Thai authorities about the June incidents, U.S. officials said.

The Thai Interior Ministry has played down the incidents, arguing that the Laotians were "economic migrants" who were not eligible for treatment as refugees in the first place.

"We instructed them they should go back because they were not qualified," one official said.

According to diplomatic sources, the Laotians, mostly Yao hill tribes people, were held three months at the Chieng Khong camp in Chieng Rai Province in northern Thailand before being put on boats and sent across the Mekong River to Laos by local Thai authorities, who had arranged the transfer with the Pathet Lao government.

There was "a lot of crying" but no violence when the Laotians realized they were being sent back to Laos instead of to another Thai camp as they had been told, one diplomat said.

Conflicting unconfirmed reports emerged about what happened to the refugees after they returned. Other refugees arriving in Thailand later told Western officials that three may have been killed and as many as 11 others imprisoned for resistance activities. Other refugee officials believe all the Laotians have been sent back safely to their homes in Luang Nam Tha Province.

In any case, refugee officials said, Laotians being held in Thai camps near the border now fear forced repatriation. In July, they said, two Laotian refugees committed suicide for reasons that are unclear. The sources said one refugee, the leader of a group of Yao, jumped into the Mekong River, after which one of his two wives poisoned herself by drinking insecticide. Accounts differ as to whether the actions were linked to the threat of repatriation or personal problems.

Western officials said they were encouraged that forced repatriations had not recurred since the June incidents. However, "the risk of further push-backs cannot be dismissed," one refugee official said.