Cambodian resistance leader Son Sann expressed disappointment yesterday with a State Department policy statement rejecting his plea for U.S. military aid.

The non-communist Khmer People's National Liberation Front headed by Son Sann formed a coalition in June with the communist Khmer Rouge to fight against Vietnam and the Vietnamese-backed government in Phnom Penh. Son Sann, on his first trip to Washington since approving this alliance, asked the Reagan administration for weapons for his non-communist troops.

In a statement before a House Foreign Affairs subcommittee Thursday morning, John C. Monjo, deputy assistant secretary of state for East Asian and Pacific affairs, testified on behalf of the administration that "We will continue our political and humanitarian support for the anti-Vietnamese coalition, but we do not plan to offer military aid to the coalition or any of its members."

Son Sann said in an interview yesterday that he was unaware of the U.S. policy statement, and that a meeting with Secretary of State George P. Shultz Thursday afternoon, several hours after Monjo's declaration, did not leave him with the impression that his request for military aid had been rejected.

"If the United States wants to oppose Soviet imperialism it has to give us help," the Cambodian leader said. He said his request was "not asking for the moon" but for rifles, mortars and anti-tank weapons for 6,000 men who have been trained as fighters but who lack weapons to join the battle in Cambodia. In addition to those men, his forces have more than 10,000 fighters under arms, he said.

"We cannot oppose the Vietnamese with our bare hands," Son Sann said. "If I see clearly that the western countries will not help us and our population will be delivered to the communists, I don't know what I will have to do."

State Department sources said the decision against supplying military aid to the noncommunist resistance was based on the belief that an eventual political settlement in Cambodia could be made more difficult by such U.S. military support. This is particularly the case because of the bitter history of military conflict between the United States and Vietnam, the sources said.

Officials said it had been made clear to Son Sann that the United States would continue to work closely with non-communist Southeast Asian countries in formulating policies and providing backing to the Cambodian resistance.