Premier Khieu Samphan of the deposed Khmer Rouge government of Cambodia has warmly thanked the United States for what he termed its diplomatic support and issued an appeal for aid from Washington and other Western capitals.

Interviewed at a shaded camp near the Thai-Cambodian border this week, the Khmer Rouge leader cited the U.S. vote in the U.N. General Assembly last September to seat the delegation of Pol Pot's Khmer Rouge government instead of the Vietnamese-installed administration of Heng Samrin.

"This is a just and clear-sight stand, and we thank the U.S. warmly," Khieu Samphan said, requesting that the support continue.

Khieu Samphan's expression of gratitude cast new light on an area of policy that the Carter administration has preferred to leave murky and largely unarticulated in the eyes of Indochina-watchers in Bangkok.

[A State Department spokesman Thursday disputed Khieu Samphan's assertion that he had U.S. diplomatic support. The Khmer Rouge government's record on human rights had been "wholly reprehensible," and U.S. support for the Pol Pot faction on the U.N. delegation issue was based on "purely technical point," the spokesman said.]

Nevertheless, Khieu Samphan -- apparently undeterred both by the past record of U.S. Khmer Rouge hostility and by the political embarrassment that the appearance of friendship with his government could bring President Carter in an election year -- suggested that the Khmer Rouge would be willing to accept military aid from Washington.

The Nixon and Ford administrations supported Lon Nol in Cambodia before he was overthrown by the Khmer Rouge in 1975. The Vietnamese put Heng Samrin in power in January 1979.

Khieu Samphan's remarks came as fighting between his forces and Vietnamese troops deployed in Cambodia appears to be intensifying. The Khmer Rouge leader argued that his guerrilla army is the key to blocking Soviet-Vietnamese plans to dominate Southeast Asia and adjacent sea lanes.

Khieu Samphan replaced Pol Pot as premier in December as part of a Khmer Rouge drive to improve the image of the government that President Carter once said had the world's worst record on human rights. A Marxist economist, he is considered a moderate at least in Khmer Rouge terms.

He called on Washington to give direct humanitarian aid to his government and to apply new political and economic pressure on Hanoi to speed the withdrawal of approximately 200,000 Vietnamese soldiers in Cambodia.

Indochina-watchers here believe that concern about Soviet influence has created in many U.S. officials an unspoken and embarrassing confluence of interest with the deposed leadership; for both parties the immediate concern is that Vietnam will consolidate control of Cambodia for Heng Samrin.

With Hanoi showing little response to diplomatic pressure and the anti-communist Khmer Serei forces small and faction-ridden, observers argue, the only means of preventing consolidation is armed force from the Khmer Rouge.

While the United States does not arm Khmer Rouge units, these experts note, neither has it used its influence in Peking to stop Chinese shipments of weapons. It also has expressed full support for the Association of Southeast Asian Nations' decision to continue recognizing the old government and supported relief shipments directly into Khmer Rouge zones.

Meanwhile Khieu Samphan and his Cabinet have launched a public relations campaign to depict their administration as an innocent and even democratic victim of foreign aggression.

In December they unveiled a "Patriotic and Democratic Front of Great National Union of Kampuchea," which supposedly would unite all anti-Vietnamese Cambodians.At about the same time, party strongman Pol Pot was relieved of the premiership.

In recent months, Khieu Samphan repeatedly has argued his case with foreign journalists and delegations at a tidy thatch camp in the hills near Cambodia's northern border with Thailand.

Countless Cambodians died by execution or starvation during the Khmer Rouge's four years in power. In the interview, Khieu Samphan acknowledged that party members had made "mistakes" during this period, but claimed that if restored, his government would implement full democracy and basic human freedoms.

Calling for U.S. help, he said, "If the U.S. and the West wait for absorption of [Cambodia] before thinking of defending Southeast Asia, it will be too late. The Vietnamese will go further -- toward the rest of Southeast Asia, the Malacca Strait, toward control of the South Pacific and Indian oceans."

Moscow is behind Vietnam's aggression, he said. It is coordinated with the Soviet Army's move into Afghanistan, which is intended to secure influence in the oil-producing Persian Gulf region.

Khieu Samphan said relief supplies sent to the Thai border by the U.S. government and American private agencies had greatly helped. But he called for deliveries directly to his government, saying it could distribute the food to people around the country.

As a great power, the United States could apply more economic and political pressure on Vietnam that would speed its withdrawal, he said. Since it is over-extended and fighting a war of aggression, he suggested, Vietnam is very vulnerable to nonmilitary pressure.

Khieu Samphan did not call directly for diplomatic recognition and weapons.

"But if the U.S. would extend military or economic aid to us," he said, "we would of course be extremely grateful. However, we dare not raise the question."

[The State Department spokesman said the United States was not considering any such aid for Cambodia, and said the U.S. position on the competing communist factions was "perfectly clear." He said the U.S. support for the Pol Pot faction on the U.N. issue was due to the fact that the competing Heng Samrin faction was "a puppet . . . a creature of Vietnamese aggression, and in our view it had absolutely no claim" on Cambodia.]

[The United States supports the concept of a neutral government in Cambodia that would "truly represent the aspirations of the Cambodian people," the spokesman said.]

Turning to military affairs, Khieu Samphan said Vietnamese troops were foundering and ultimately would be forced to withdraw. Nevertheless, Hanoi's forces appeared to be stepping up pressure on Khmer Rouge strongholds near Thailand. Positions in the Phnom Malai hills south of the Thai border town of Aranyaprathet have been shelled almost daily for the last month.