Soviet pilots have flown many of the transport planes ferrying Vietnamese troops to western Cambodia, according to two Vietnamese deserters. One of the men said that last July he saw 50 to 70 Soviet technicians working on aircraft at Siem Reap airport.

The deserters also said northern Vietnamese did not trust their southern countrymen and assigned them noncombat roles in Cambodia. They said even pro-Vietnamese Cambodians seemed to be considered more reliable than the southerners.

The two Vietnamese were interviewed with a third deserter at a Thai military prison near the town of Aranyaprathet. Early last month the three men crossed into Thailand together and were taken into custody. Their names have been withheld to protect relatives in Vietnam.

Thailand currently holds about 160 Vietnamese deserters, most of them draftees from southern Vietnam. A few even served in the old South Vietnamese armed forces. Military analysts see their conscription as a sign that the Cambodian venture and the confrontation with China have seriously stretched Hanoi's manpower.

[In Washington, Pentagon officials said they had been aware for about a year that the Soviets were flying into Cambodia. But the report of Soviet technicians based on the ground in the northwestern Siem Reap area appeared to go beyond previously supposed levels of Soviet involvement in Vietnam's Cambodian campaign.]

Large-scale Vietnamese involvement in Cambodia began in October 1978 with the drive on Phnom Penh that overthrew the Chinese-backed Pol Pot government three months later.

Vietnam's actions in Cambodia sharply increased tensions between China and Vietnam, culminating in the border war last January between the two countries. The Soviet Union has supported Vietnamese policy, and this in turn has exacerbated already existing Sino-Soviet differences.

H., 21, said he landed at Siem Reap airport in July this year aboard a U.S-built C123 transport, apparently one that the communists captured from the old South Vietnamese armed forced in 1975. He saw the pilot was a Caucasian and other soldiers told him the man was Russian.

H.'s unit remained in Siem Reap for further training. During this time he observed 50-70 Russians working on airplanes. They did not leave the airport perimeter, however, he said.

N., 25, was flown to Battambang airport in October this year. His plane was also piloted by a Russian, he said, and was also a C123.

Dressed in pants, fresh denim shirts and wearing cumbersome leg irons, the three Vietnamese talked freely of their time in Cambodia. Combat troops are all northerners, they said, and their morale is high. But southerners, used as laborers and forbidden to carry weapons, are sick of the war.

Relations with Cambodian civilians, meanwhile, are generally peaceful. But the Vietnamese are resented because they eat better food, occasionally rob people, and openly control the Heng Samrin government.

Their stories did not conflict significantly with information that refugees and intelligence sources have provided on Vietnamese forces in Cambodia. Believed to number between 178,000 and 2000,000 men, the troops are stationed in towns and large villages and along major roads.

D., 24, served in Cambodia the longest of the three. Given only one month of training in Vietnam, he was sent by truck in October 1978 to the area around Snuol Town, bordering on Vietnam. On climbing down from the truck he was surprised to learn from other soldiers that he was in Cambodia.

Two months later the Vietnamese army began the westward push that captured Phnom Penh and overthrew the Khmer Rouge government headed by Pol Pot. D.'s unit followed close behind crack North Vietnamese troops as they drove the Khmer Rouge westward He eventually reached Battambang Province, bordering on Thailand.

He worked as a cook and laborer. At times he was assigned to move the Vietnamese dead and there were many. "For the first six months we returned the bodies to Vietnam. After that we began burying them in Cambodia," he said.

N. and H. arrived in Cambodia in the second half of 1979. They were apparently sent in as reinforcements for the dry season against Khmer Rouge and right-wing Khmer Serei guerrillas.

Like D., they were assigned as rearechelon laborers. All three soldiers maintained they were never allowed to carry weapons, even though they were trained in Vietnam to use the AK47 rifle.

The army did not trust people raised under the American-sponsored governments in South Vietnam, they said. They were normally put into mixed units -- one southerner for every two northerners -- in a apparent attempt to enforce discipline.

Pro-Vietnamese Cambodians seem to be considered more reliable than southerners. The deserters said the Vietnamese have organized a few small units of armed Cambodians. They carry old weapons captured from the Khmer Rouge. They do not actually fight, but occupy areas the Vietnamese have already cleared.

Many southerners find life in the field intolerable, they said.Moreover, they do not believe political cadres who say Vietnam is helping out the Cambodian people. "When I saw all the corpses," H. recalled, "I began to think maybe we're not here to help Cambodia, we're here to take it over,"

But there were few discernible cracks in the morale of the northerners, who comprise all the combat troops, they said. "The North Vietnamese never get tired," H. said. They'll fight until they get what they want."

The three men said they had never actually seen Vietnamese troops mistreat civilians. But they knew that civilians often hid when troops entered a village out of fear they would be robbed or killed.

In Battambang, the soldiers received delieveries of rice from the Army. They generally had more to eat than the civilians, who had to walk to the Thai frontier and carry back food provided by the U.N. and Red Cross or bought in black markets.

D. was the only one of the three who had seen the Heng Samrin government distribute rice. In late November a truck loaded with rice arrived from Siem Reap and the village headman passed out two cupfuls to every family.