A thin straggle of refugees trudges westward, carrying bundles of clothes, babies, cooking pots and chickens. Assisted by young Khmer guerrillas in olive-green fatigues, they move slowly down the dusty track toward an assembly point on the Thai border.

In the opposite direction, Chinese Army trucks loaded with militia women and crates of ammunition jolt eastward in clouds of swirling dust into the Cambodian jungle.

From jagged limestone crags that rise sharply along the southern horizon comes the distant boom of artillery explosions interspersed with the sharper thud of mortar fire that marks the progress of steadily advancing Vietnamese troops.

For two weeks sporadic artillery fire has been probing the outer defenses of the Khmer Rouge's border redoubt of the Phnom Malai Mountains south of the Thai border town of Aranyaprathet. But since Jan. 30, Vietnamese forces, pressing on with an already dramatically successful border offensive, have begun pushing into the southern flank of the Phnom Malai, the biggest and, in the view of some analysts, the most important base area held by Khmer resistance troops battling Hanoi's forces in Cambodia.

The move, analysts say, appears to mark the opening of a full-scale assault on the area.

Initial Vietnamese thrusts into the heights 10 miles south of here, backed by often heavy supporting artillery fire, already have seen Khmer Rouge guerrillas ceding hilltop positions and falling back to the north. The shellings also have prompted the beginnings of what analysts say could be another major exodus of Cambodian refugees into Thailand.

Following Vietnamese artillery barrages on Jan. 30, about 4,000 civilians were evacuated from the Khmer Rouge-administered village of Chakrey, the southernmost settlement in the Phnom Malai area.

"We are moving out the old, the young and some women," said a senior Khmer Rouge officer at Khao Din Camp, a few miles north. "For the present, most of the people are staying, but if the Vietnamese attack in strength, we will have to move the civilians out altogether."

Fighting south of here already has spilled over into Thailand, according to Thai Army field sources. Vietnamese shells have fallen on Thai soil and company-sized units have crossed the border in pursuit of Khmer Rouge troops retreating northward along the Thai side of the rugged and often ill-defined border.

Vietnamese incursions into Thailand's Trat Province, near the guerrilla base of Boral farther south, have sparked clashes with Thai forces over the past three days. But the fighting near here has not brought Thai and Vietnamese troops into direct collision yet.

Comprising a string of civilian border settlements as well as military camps and supply dumps farther inland, the base area is estimated to house at least 35,000 civilians, according to a recent estimate by the U.N. Border Relief Organization. But relief sources in Aranyaprathet, who have had little access to the area in recent years, say they believe the figure could be substantially higher.

The area is defended by approximately 7,000 to 10,000 guerrillas -- less than one-quarter of the Khmer Rouge's nationwide strength of 35,000 to 40,000. Since June 1982, the communist Khmer Rouge have been allied to the noncommunist Khmer People's National Liberation Front, numbering about 15,000 troops, and a nationalist faction loyal to Prince Norodom Sihanouk that fields slightly less than 10,000 men.

Hanoi, after a series a crushing assaults from mid-November through early January aimed mainly at camps of the National Liberation Front along the Thai border, has turned its attention toward the militarily more menacing Khmer Rouge. Following the fall of the National Liberation Front's headquarters at Ampil on Jan. 8, Vietnamese forces have been concentrating around the Phnom Malai area in preparation for what analysts say could be the high point of Hanoi's most punishing dry-season campaign of the six-year-old war.

Currently massed against the base are two full Vietnamese divisions and elements of at least two others. The concentration, backed by artillery and armor, is the largest force the Vietnamese have assembled against a single focus of resistance since the 1978 invasion that swept the Khmer Rouge from power and installed the pro-Hanoi Heng Samrin government.

Previous Vietnamese efforts to penetrate the area proved bloody and inconclusive. Unlike their other border strongholds, rugged terrain and the size of the Phnom Malai zone permit the guerrillas greater room for maneuver.

But the sheer mass of manpower and hardware now poised around the area suggests that the Vietnamese, buoyed by their successes to date, are determined to drive both the guerrillas and their civilian support base from the Phnom Malai.

While giving ground before advancing Vietnamese in the south, the Khmer Rouge so far have concentrated resistance against larger Hanoi forces massed against the base's northern flank. During the past week the guerrillas have launched preemptive raids on Vietnamese artillery bases and ambushed convoys moving up men and supplies along approach roads.

But Thai military sources say they believe the pressure will begin to bite when Vietnamese forces start a concerted pincers attack, mounting assaults against guerrilla positions in the north while continuing their drive against the base's southern flank.