Official U.S. warnings in recent weeks of an imminent Vietnamese offensive in Cambodia appear to have been false alarms triggered not by new information but by the Carter administration's determination to signal continuing concern about the war in Cambodia, diplomats here report.

State Department spokesmen in Washington have twice since mid-December warned of Vietnamese plans to attack the huge settlements of Cambodian refugees along the Thai-Cambodian border. One such warning included a precise date of the weekend of Dec. 22 for a Vietnamese offensive, which U.S. officials then said could spill over into Thailand and undermine the government here.

The attacks have not materialized, and diplomats here play down the siginificance of the new intelligence the State Department said it had obtained. The Washington briefings were intended, these sources said, primarily as a reminder that huge numbers of Cambodians are still massed on the border and living under the guns of a hostile army.

U.S. officials are concerned that public and official attention in Washington has swung almost completely to Afghanistan and Iran in recent weeks.

Last week's widely publicized renewed warning about a Vietnamese attack came as many Indochina analysts here had concluded that the Vietnamese do not consider the camps a threat and do not intend to move against them at all.

These analysts appear to be split over the effect of Washington's decision to seek maximum publicity on the possibility that the Vietnamese could attack the camps. Some feel that the campaign limits American room to maneuver with the Vietnamese for a decrease in tension along the Cambodian-Thai frontier.

Last summer Thai intelligence and Western diplomats began reporting a buildup of Vietnamese forces and equipment in western Cambodia.

It appeared to be preparation for a concerted push in the dry season to secure the frontiers for the Heng Samrin government in Phnom Penh. A main goal supposedly was to stamp out all remaining armed resistance both from rightist Khmer Serei soldiers, who control most of the estimated half million refugees on the border, and from soldiers loyal to the ousted Khmer Rouge government of Pol Pot.

The rains ended in the fall. Scattered fighting began, but not the expected general offensive. There were repeated false alamrs, however, climaxed by Pentagon analysts saying they thought the fighting would start Dec. 22, to mark the Vietnamese armed forces day.

The report was taken seriously by many relief agencies and some began stockpiling medical supplies at the border and drafting evacuation plans.

Since Dec. 22 passed without incident, later predictions that an offensive is coming at a particular time have been greeted with skepticism.

Some diplomats now question whether the Vietnamese intend at all to move on the Khmer Serei camps, where most of the estimated half million refugees on the border live. The camps, they say, do not pose a significant military threat to the Vietnamese while they do provide an important flow of income in the form of rice that is hauled to the Cambodian interior.

Most analysts here dismiss the right-wing Khmer Serei as warlords and bandits.While they profess to be committed to reconquering Cambodia, analysts say, the Khmer Serei spend much of their time making money off refugees and feuding with other anti-Vietnamese factions.

Diplomats who question whether the Vietnamese intend to move against the border camps say that such an attack would serve no purpose. It would close off a valuable source of food, they say, and achieve no tangible military advantage by neutralizing the Khmer Serei. Attacking refugees also would inflame world opinion, they say, possibly bring retribution from China, and make the refugees that much more wary of returning to Vietnamese-controlled villages later.

Most analysts here still feel, however, that the Vietnamese cannot afford to ignore the Khmer Rouge zones and that an offensive, perhaps limited to repeated small engagements, is unavoidable.

The Khmer Rouge have an estimated 25,000 soldiers under arms still receving ammunition from China. The troops are by most accounts well-fed, healthy, and highly motivated, and could seriously disrupt Vietnam's plan for Cambodia.