Cambodian resistance leader Son Sann has come to Washington asking the United States to keep what he calls its gentleman's agreement and supply his noncommunist army with weapons to fight the Vietnamese occupation troops in Cambodia.

In an interview here Son Sann publicly admitted for the first time that he and his Khmer People's National Liberation Front reluctantly formed a coalition with the murderous Khmer Rouge because the West offered them little choice.

"During the first three years of fighting the Vietnamese we received nothing from western countries," he said. "Then in 1982 the western nations let us understand we wouldn't receive aid unless we accepted to join a coalition with the Khmer Rouge because the U.N. voted to seat the delegation of Democratic Kampuchea," the government of the Khmer Rouge.

The United States and other western nations supported the seating of the delegation of the Khmer Rouge, believed to have killed almost 2 million people during its four years in power in Cambodia, in an effort to force the withdrawal of Vietnamese occupiers from Cambodia.

Now more than a year has passed without military aid from the West, Son Sann said. As before, China arms the Khmer Rouge and gives some arms and ammunition to Son Sann's group. This year Singapore discreetly donated about 3,000 rifles to the Khmer People's National Liberation Front. The United States gives Son Sann indirect humanitarian aid through international relief organizations.

"One must give food to a sick man but also medicine," said Son Sann about the U.S. aid position. "The right medicine is the liberation of Cambodia."

When Son Sann meets with Secretary of State George P. Shultz this week he will come with a shopping list: 5,000 automatic rifles and ammunition plus mortars, antitank weapons and explosives.

"The Khmer Rouge are still receiving much more aid than we are," Son Sann said. "And they don't begin to have the recruits we have."

Son Sann said his forces now number 12,000 armed and 5,000 unarmed combatants, hence the urgent request for rifles. The Khmer Rouge have an army of about 30,000 soldiers. Both are fighting a Vietnamese occupation force of about 150,000 that has controlled Cambodia since invading it in 1979.

Son Sann's army has grown from a force of 2,000 soldiers three years ago while the Khmer Rouge have declined from a strength of about 50,000, western sources estimate.

Son Sann argued that now is the time for the United States to give his troops support, saying the Vietnamese are weary and looking for a political settlement. Vietnam, however, has shown little flexibility in negotiation; some observers believe Hanoi has decided to seek a military solution.