Khmer Rouge forces poured across a narrow neck of Thai territory today in a desperate effort to reach a mountain sanctuary back in Cambodia before being cut off by Vietnamese-led troops who invaded Cambodia in January.

Western diplomats here said the exodus of the Khmer Rouge Followers of Pol Pot - herding tens of thousands of ragged and hungry civilians with them - means the end of their efforts to maintain a nationwide resistance to the invasion forces during the approaching rainy season.

While the Khmer Rouge force escaped from one Vietnamese trap by cutting across 10 miles of Tailand, military analysts here said the relief in only temporary.

They expect the more mobile Vietnamese forces to wheel around and speed across the Cambodian plains to cut off the retreating Khmer Rouge columns before it reaches Pol Pot's major bases in the Cardamom Mountains in Cambodia, 40 miles southeast of the Thai border.

The retreating Chinese-backed Khmer Rouge column 50,000 to 100,000 persons - including both teen-age troops and villagers who pleaded with Westerners to help them stay in Thailand - are the remnants of two shattered Cambodian commands who fell back to the Thai border.

They attempted to make a stand at the strategic border town of Poipet, where they thought they had a chance to secure a foothold.

But the Vietnamese invaders, riding in U.S. equipment captured when Vietnam fell, moved too quickly. Five to six divisions, reinforced with armor, were dispatched by Hanoi early this month to clean up the Khmer Rouge resistance before the rains start in May and cut their retreat in Pol Pot's mountain stronghold.

This Vietnamese force has sliced communications betwwn the Khmer Rouge forces in the northern and southern parts of Battambang Province and for the past two weeks a Vietnamese two-pronged pincer movement has trapped Pol Pot's retreating soldiers in a narrow cul-de-sac that juts into Thailand 10 miles southwest of Poipet.

For the past three days of Cambodian have been retreating out of that trap through Thai territory. They formed a ragged band, stretching as far as the eye could see. The leader of one group rode a badly worn motorcycle and other soldiers moved up and down the line on sturdy Cambodian ponies. Most of the refugees trudged along, but some were mounted on their water buffaloes.

The caravan is apparently typical of the nomadic Khmer Rouge resistance, which is designed to deny both rice and people to the Vietnamese invaders. Some Khmer Rouge leaders have told Thai soldiers that they corssed the whole of Cambodia in a slow migration, dragging villagers and as much rice as they could carry with them.

Until today, the Vietnamese have held back their artillery fire, apprently worried about shelling Thai territory. Today the Vietnamese fired 30 mortor rounds at the retreating columns, but no injuries were reported.

Senior Thai Army officers are concerned that Hanoi, which has already accused Bangkok of supplying the Khmer Rouge, will try to follow the retreating column.

According to sources here, the last thing the Thais want is to fight with either side in the Cambodian war. CAPTION: Map, no caption, By Dave Cook-The Washington Post