The first signs of a new phase in the war between Vietnam and Cambodia are discernable to analysts here, as persistent guerrilla harassment by forces loyal to deposed premier Pol Pot is beginning to expose the weakness in Vietnam's armored conquest of Cambodia.

The superior force of Vietnam's mobile armore managed to roll up key highways and secure provincial towns and airports in an unexpected 20-day blitz. The speed of the assault allowed the new pro-Hanoi Cambodian government headed by Heng Samrin to claim control of the country shortly after the occupation of the capital of Phonom Penh on Jan. 7.

But the Vietnamese victory has proven partial at best. The fact that the Khmer Rouge evacuated the towns three years ago allowed Pol Pot's forces to years the country's strategic and administrative centers without giving up control over a significant proportion of the countryside.

Moreover, there were few major engagements during Vietnam's lightning assault. Sizable Khmer Rouge units either were by-passed by the motorized Vietnamese columns, or managed to evacuate provincial capitals ahead of the enemy assault.

As a result, the Khmer Rouge have regrouped in the Vietnamese rear with apparently considerable freedom of movement in the countryside. The roughly 100,000 Vietnamese forces estimated to be inside Cambodia may overwhelm their enemy both in numbers and firepower. But, according to one veteran Indochina analyst, they remains as road-bound as the Americans were in Vietnam.

With the Vietnamese holding an almost empty strategic shell and the Khmer Rouge moving among the population in the countryside, analysts here discern a new and more portracted phase in the war -- the battle for winning over the people.

Admittedly dealing with only meager intelligence far from the battle scene, analysts here sketch the following picture:

The Khmer Rouge forces are concentrated in three areas of the country. An estimated 30,000 Pol Pot loyalists are thought to be operating in the flat land east and southwest of the lower Mekong where there is little jungle cover but large concentrations of people and rice.

An unknown but significant portion of the two divisions defending the northeast is thought to have slipped out of Kratie and Stung Treng to the west bank of the Mekong and are now operating in the northern provinces of Preah Vihear.

A Khmer Rouge officer who recently crossed into Thailand reportedly told Thai intelligence that he personally saw former president Khieu Samphan leading a force of 8,000, including 200 Chinese advisors. Western analysts consider the numbers exaggerated.

The third, and perhaps most vital area in the long run, is the mountainous Cardamom Range, ideal guerrialla terrain running from the Thai border down the coastal province of Koh Kong. This is thought to be the center where the Cambodians laid up stocks of food and ammunition on Chinese advice some months ago. It is also the target of naval, air and amphibious assaults last week by the Vietnamese seeking to seal the coast on the Gulf of Thailand against the possiblity of Chinese supplies coming in by sea.

Reports have increased over the past week of guerrilla actions in all these areas. Small Khmer Rouge units apparently are probing weaknesses in the Vietnamese rear and attacking the slender security forces left behind to hold provincial towns after the main Vietnamese rear and attacking the slender security forces left behind to hold provincial towns after the main Vietnamese invasion column moved on.

Over the weekend, the Khmer Rouge are reported to have blown bridges along the critical supply arteries ot the west, Highways 5 and 6. There are also sings that the Khmer Rouge are massing a strike force near the western provincial capital of Battambang which the Vietnamese entered on Jan. 14 after the Cambodian forces apparently pulled out. Observers view last week's battle over the country's only deep-water port, Kompong Som, as indicative of the type of see-saw pattern likely in coming weeks in key contested areas.

Analysts say that none of this activity is likely to change the strategic balance in Cambodia. If Khmer Rouge harrassment continues, however, it will both increase the cost to the Vietnamese of keeping overstretched lines of supply open, particularly to forward positions in western Cambodia, and slow the process of extending the control of the new Heng Samrin government into the districts and villages where most Cambodians live.

The Khmer Rouge are not without their own problems running a guerrilla operation in widely isolated units, vulnerable to air attacks and defections. Nevertheless, the anticipated mass exodus of refugees and Pol Pot loyalists has not yet developed as the Vietnamese get closer to the Thai border.

In this late harvest season anyone with a gun can get rice in the countryside. So how long the Khmer Rouge can keep up the contest depends on their stocks of ammunition, on Chinese intention and ability to resupply their allies either through the territory of a reluctant Thai government or by sea, and the political success of the new government in Phnom Penh.

After three years of revolution and brutality, the new Cambodian government's promises to restore such concept's as the family, religion, personal freedomas and the market is thought to have wide popular appeal. But the need for an estimated 100,000 kilograms of rice per day to feed the Vietnamese army in Cambodia may compromise the new government's projected liberalism.

Given the scarcity already in Vietnam, observers here suspect this rice will soon have to come from within Cambodia, perhaps procured at gunpoint.

The Hano-backed government may promise freedom, but whether the Cambodian vilager will surrender his rice to feed an occupying army or support the perpetrators of harsh polices under the Khmer Rouge banner of nationalism remains one of the key issues in this new phase of the Cambodian war.