A large-scale Vietnamese offensive aimed at wiping out Pol Pot's forces in Cambodia is imminent and may have begun today, according to U.S. officials.

The offensive is certain to drive more Cambodian refugees into Thailand, to worsen the famine conditions inside Cambodia and to put new pressures on the Thai government.

There has been a tremendous increase in Vietnamese military activity in Cambodia, according to U.S. officials. Within the last 24 hours, large units have begun moving in several parts of the country, including western Cambodia, where Pol Pot's strength is greatest.

By U.S. estimates, there are between 170,000 and 200,000 Vietnamese troops in Cambodia. Pol Pot, who was overthrown last January following the Vietnamese invasion, is believed to have 20,000 to 30,000 soldiers.

The offensive is timed for the end of the rainy season, which began three months ago. The rains make it difficult for the mechanized Vietnamese units to travel. This year's rainy season was unusually short, so the offensive is beginning earlier than analysts at first expected.

China firmly supports Pol Pot and sees the Cambodian situation as the beginning of a protracted guerrilla war. There is no indication China will intervene directly, but Chinese supplies have been reaching Pol Pot, some by sea and others through Thailand.

The Vietnamese are armed better than Pol Pot and are expected to inflict heavy casualties during the offensive.

A major concern to Thailand, the ASEAN nations of Southeast Asia and the United States is the possibility that Thailand will be drawn into the conflict if, as is likely, some of Pol Pot's forces flee across the border to take sanctuary in Thailand.

The Thais fear Vietnam's military power and have no desire to see the Vietnamese-installed Heng Samrin government in Phnom Penh establish complete control over Cambodia. Thailand, with the other ASEAN nations, played a leading role at the U.N. General Assembly last week in the debate that ended with a vote to allow Pol Pot's representatives to retain Cambodia's U.N. seat.

In that debate, U.S. Ambassador Richard Petree declared Friday that the Vietnamese dry-season offensive already had begun.

A considerable part of Secretary of State Cyrus Vance and his staff's first day here was devoted to the Cambodian problem. One subject of discussion was how to get international food aid into the country, where tens of thousands of people are reported to be starving.

Vance discussed aid with U.N. Secretary General Kurt Waldheim and bilateral talks with other nations are continuing.

Sinapore Ambassador Tommy Koh told reporters the ASEAN nations will tell the United States this weekend that any international aid must be carefully monitored so that it cannot be diverted for use by the Vietnamese armed forces. He admitted that such monitoring could be extremely difficult to arrange.

Cambodia is a major test of will for China and Vietnam. China invaded Vietnam last February in retaliation for Vietnam's invasion of Cambodia. Peking has spoken of "teaching Vietnam a second lesson." A second invasion would be more costly to China because Vietnam has reinforced its positions along the China border.

Laos also is involved in the Sino-Vietnamese confrontation. Vietnam dominates the Lao government, but there are reports of increased Chinese activity in support of insurgent groups.