The United States expressed concern yesterday about reports of large Vietnamese troop deployments and possible attacks on refugee camps along the Thailand-Cambodia border.

A statement issued by the State Department said the Vietnamese military movements pose "a potential threat to the security of Thailand," a U.S. ally.

State Department officials said they have no hard evidence that a Vietnamese attack on refuge centers is imminent, but that a variety of recent indications has raised the level of their concern.

Among these, they said, is the report of a defecting former official of the Vietnamese-backed Heng Samrin regime in Phnom Penh that attacks are likely before the end of this month. A step-up in Vietnamese military operations and intelligence patrols as well as a notable increase in denounciations of international feeding operations in the border area are other indications that were cited.

Another worrysome element, which was not mentioned in the State Department announcement, is new concern about the internal stability of the government of Thai Prime Minister Chamanan Kriangsak. In an unusual incident, Kriangsak suddenly canceled an official visit to Europe last week amid a labor dispute and reports of a split in the ranks of the government.

Both Vietnam and its sponsor, the Soviet Union, repeatedly have informed the United States and other nations through diplomatic channels that there will be no attack on Thailand, according to U.S. officials. At the same time, Vietnam has continued to charge Thailand with complicity in aiding and feeding anti-Vietnamse insurgents near the Cambodian border.

The State Department specifically called on Vietnam and the Soviet Union "to refrain from any action which would threaten Thailand's security and integrity or endanger the well-being and safety of the noncombatants in the refuge concentrations along the border."

A Vietnamese military offensive against the Cambodian guerrilla forces led by Pol Pot has been expected since late December. (Pol pot has since been replaced as political leader of the insurgency, but remains the military chief). Although Vietnamese military sweep operations have intensified, State Department officials said the previously anticipated level of combat has not developed yet.

According to U.S. intelligence reports, about 50,000 Vietnamese troops are deployed along the Thailand-Cambodian border. Slightly less than 500,000 Cambodia refugees are reported encamped along the border, with 150,000 other refugees in "holding camps" in Thailand's interior.

Officials expressed particular concern about two large refugee camps, Nonmakwun and Nonsamet, which are north of the area where most of the fighting between the Vietnamese and Pol Pot forces has been taking place. The two camps together are estimated to contain 240,000 to 290,000 refugees.

Asked about possible motivations for an attack against the refugee centers, the State Department officials cited Vietnamese opposition to the international feeding operations at the camps and along the border. A substantial part of the food is taken back into Cambodia, including some food that is placed at regular pickup points for this purpose.

Food represents political influence and possibly even control in impoverished Cambodia, an official noted.

Another possibility is that Vietnamese troops may try to seal the Thailand-Cambodian border to improve their position against the insurgents, according to officials. They said recurrent reports in Thailand had spoken of such an effort.