On the way to the port of Kompong Som 135 miles southwest of here, a heavy truck barrels down the road with a motley group of passengers. Bouncing along in the back are a dozen Cambodian civilians, a few Vietnamese soldiers wearing their characteristic pith helmets, and a pig.

In another country under military occupation -- Afghanistan, for instance -- such a mixture of foreign troops and local inhabitants would be a rare sight indeed. But here it is not at all uncommon.

Despite a history of antagonism between Vietnam and Cambodia and reputed morale problems in the Vietnamese Army, what can be seen of the Vietnamese military presence during a tour through Cambodia appears relatively benign.

It is, in fact, not the troops, but a more general perception of intentions by Hanoi to exploit Cambodia that is likely to strain the countries' relationship, knowledgeable foreign residents here believe. For while it is apparent after 21/2 years of occupation that the Vietnamese did Cambodians a favor by ousting the Khmer Rouge government, it is equally clear that in doing so, Hanoi was primarily pursuing its own interests, not those of the Cambodians.

During a week-long visit to Cambodia, there was no sign of any great animosity toward the Vietnamese troops in areas under their control. More often than not, the soldiers seen walking along city streets and provincial roads were unarmed. They live a Spartan life, with food and shelter generally no better than that of most poor Cambodians.

Although there were early reports of looting when Hanoi's forces invaded in December 1978 and ousted the Khmer Rouge government of Pol Pot, Western aid officials generally credit the Vietnamese troops with exemplary behavior.

Possible exceptions to this record are cited by U.S. officials in Bangkok, who quote many Cambodian refugees arriving at the Thai border in the last few weeks as complaining about Vietnamese misdeeds in various provinces, including robberies and rapes. The officials are not sure how much of the accounts might be exaggeration by people seeking to emigrate.

According to the officials, most of the complaints are about Vietnamese soldiers serving in remote or embattled areas where they are under pressure or have to scrounge for provisions because of poor logistics.

But while the Army appears comfortable, some traces of strain between Cambodia and Vietnam already have been reported because of the Phnom Penh government's budding relations with Moscow, although the government here denies any such problems.

Asked about reports of Vietnamese annoyance over the growing Soviet role here, Vice Foreign Minister Hor Nam Hong said, "These rumors are without foundation. Both are socialist countries that help us without ulterior motives or self-interest."

His remarks tended to put the Soviets on a par with the Vietnamese, however. On ties with Moscow, he noted, "It was Vietnam and the U.S.S.R. that first brought us aid. Our relations with the U.S.S.R. are very good and will continue to be in the future."

According to a foreign resident with close contacts among Cambodians, some strains have appeared because of Vietnamese efforts to discourage any other foreign participation -- including Soviet or East European -- in teaching and training in Cambodia, especially of soldiers.

Nevertheless, the Phnom Penh government has been sending a growing number of officers, cadres and students to Moscow for training. Even grade school children are sent abroad by the hundreds, mostly to the Soviet Union, for years of instruction.

The Soviets have also increased their own presence in Cambodia. About 400 Soviet personnel are currently estimated to be in the country, compared to about 40 early last year. A key element is a crew of Soviet dockers and technicians at the port of Kompong Som who play a vital role in handling aid shipments and operating port facilities.

For the Soviets, Cambodia seems to have become a big marketplace. Soviet personnel and their wives can often be seen leaving from Phnom Penh airport with Japanese stereos, electronic equipment and various appliances bought on the capital's free market, where the goods arrive after being smuggled in from Thailand.

In contrast to this official desire to develop closer ties with Moscow independently of the Vietnamese, there is evidence of a fair amount of anti-Soviet feeling among the populace. Westerners mistaken for Russians are regularly subjected to anti-Soviet catcalls, and Soviets on occasion have been the targets of attacks.

In November, two Soviets were killed and several injured when attackers believed to be Khmer Rouge opened fire on their minibus at Elephant Pass in a mountainous area between Phnom Penh and Kompong Som.

In a more recent and mysterious incident, according to Western sources, a car carrying Soviet advisers came under fire on the outskirts of the capital. The Soviets reportedly were unhurt, but the Cambodian driver was hit.

"People here are more pro-Western than pro-Russian, and Americans are favorably looked on," said a European aid agency director. He confirmed that the Soviets were developing their own lines to Cambodia, noting that the Soviet and Vietnamese embassies in Phnom Penh "act very independently of each other."

However, the Vietnamese maintain tight control here through advisers in all ministries who act on orders from Hanoi, according to aid officials, and the Cambodian Army appears to be little more than a motley adjunct to Vietnamese units.

"Any basic political decision is made in Hanoi," the agency director said, adding that all security matters are referred to Vietnam.

This dependence on Vietnam has been among the complaints cited by government officials who have fled to Thailand in recent weeks. Another potential source of friction, Western officials said, is the reported settlement of Vietnamese civilians in Cambodia.

"More and more Vietnamese are settling here," said a foreign resident with broad experience in the country. "You see more Vietnamese civilians doing business in the provinces."

He said he had heard accounts of farming settlements established by Vietnamese families in areas of the eastern provinces of Kratie and Stung Treng that are off limits to foreigners.

This source said he was convinced that a key Vietnamese motive in continuing to occupy Cambodia was to secure future food supplies -- a Cambodian "rice basket."

"This country is not a Vietnamese satellite," he said. "It's a colony. Vietnam is a pure colonial power."

Both Phnom Penh and Hanoi like to portray the Vietnamese intervention as intended solely to rid the country of the "genocidal Pol Pot regime." But the principal motive cited by Western diplomats in Vietnam was to secure Vietnam's Western border against increasing attacks and provocations by the Khmer Rouge in 1977 and 1978 and to halt the development of a hostile, pro-Chinese presence next to Vietnam's southern Cochinchina region, which is inhabited by more than a million ethnic Chinese.

In its apparent effort to turn Cambodia into a reliable client state, Hanoi has been promoting Cambodian officials who have long associations with Vietnam. The leading such official is Pen Sovan, 45, the regime's strongman. He holds the posts of People's Revolutionary Party general secretary and premier. He lived in Vietnam for 25 years, speaks fluent Vietnamese and has a Vietnamese wife.

But for all that, some Western officials here believe Pen Sovan is likelier to steer a course more independent of Vietnam than President Heng Samrin, a former Khmer Rouge major who defected to Vietnam in early 1978.

Pen Sovan is considered the stronger personality and more pragmatic, officials said.

"I don't think his stay in Vietnam necessarily made him into a creature of Vietnam," one relief agency director said of Pen Sovan. "He will not be an enemy of Vietnam, but he won't be a slave either."

For now, he and other Western aid officials agreed, the overriding feeling here toward Vietnam is one of gratitude for ending the horrors of the Khmer Rouge. Whether that sentiment may eventually be overtaken by the emerging resentments, they said, cannot yet be forecast.Next: From famine to economic brinkmanship.