Thai forces, Cambodian guerrilla groups and more than 300,000 Cambodian and Thai civilians are bracing for another seasonal offensive by Vietnamese troops along the Thai-Cambodian border amid conflicting reports about Hanoi's intentions this year.

As Vietnam approaches the seventh anniversary of its December 1978 invasion of Cambodia, the prospect of a bloody annual rite -- the dry-season offensive against Cambodian resistance groups -- has aroused more than the usual fear because of the high vulnerability of Cambodian refugees encamped on the Thai side of the border.

In recent months Thailand has been moving to consolidate refugee settlements into three main "evacuation sites." This has made them easier to control and administer, but more vulnerable and harder to move to safety in the event of a Vietnamese attack, western relief officials say.

Contributing to the Cambodians' anxieties have been reports from the Thai military and Cambodian resistance groups of a Vietnamese buildup in preparation for the November-to-May dry season.

But the Vietnamese insist that they have no intention of attacking along the 450-mile Thai-Cambodian border this year or of hitting the refugee camps on Thai territory.

Some western diplomats and Thai military sources are inclined to take Hanoi at its word this time, pointing out that the main resistance problem for Vietnam this year is in the Cambodian interior.

The largest of the three resistance groups, the communist Khmer Rouge, has stepped up its activities inside Cambodia in recent months, ambushing Vietnamese troops and recruiting Cambodian youths in rural areas, according to intelligence sources. Khmer Rouge units have been sighted less than 10 miles from the Cambodian capital, Phnom Penh, the seat of the Vietnamese-installed government of President Heng Samrin. Last month, Khmer Rouge rockets landed in the capital's northern suburbs about three miles from the city center, the sources said.

According to a Hanoi-based western diplomat, the Vietnamese now admit that there are up to 10,000 Khmer Rouge troops inside Cambodia. Western estimates generally put Khmer Rouge strength at 30,000 to 40,000 guerrillas.

By contrast, two noncommunist resistance groups, the Khmer People's National Liberation Front led by former premier Son Sann and the National Army of Prince Norodom Sihanouk, have not done as well in mounting guerrilla operations in the interior. Particularly disappointing to its backers has been the performance of Son Sann's group, the largest noncommunist resistance group, which has been practically paralyzed by internal squabbling since the last Vietnamese dry-season offensive.

That offensive, which ended earlier this year, wiped out every major resistance base on the Cambodian side of the border with Thailand, forcing nearly a quarter of a million Cambodians to flee to evacuation sites on Thai territory. In a departure from previous tactics, the Vietnamese this year remained in their forward positions along the border through the rainy season in an effort to cut guerrilla infiltration into Cambodia.

Vietnamese military and government officials told western visitors recently that there is no need for a big campaign on the border this year because, as one Hanoi official was quoted as saying, "there is nothing to launch a major offensive against."

Instead, some Cambodia-watchers here expect increased efforts in the border area to stop infiltration of resistance fighters and supplies combined with operations inside the country.

According to Lt. Gen. Tran Cong Man, the editor of Vietnam's Army newspaper, "Last year our intention was to attack bases of the three [guerrilla] factions along the border. This year it is to wipe them out in the interior," he told reporters in Hanoi last week.

However, resistance groups have reported an influx of Vietnamese reinforcements and Soviet arms into Cambodia in preparation for what they say will be a major border offensive.

Son Sann's group said this week that the Vietnamese have moved about 12,000 new troops, 70 tanks and a number of artillery pieces into western Cambodia. Total Vietnamese troop strength in the country has been estimated at between 160,000 to 180,000.

The Khmer Rouge said earlier this month that two Soviet ships loaded with tanks and artillery had arrived at the Cambodian port of Kompong Som on Oct. 24.

Such reports are ominous for the nearly 240,000 displaced Cambodian civilians living in camps on the Thai side of the border and the estimated 80,000 Thai villagers inhabiting those areas. About 200,000 of the Cambodians now are concentrated in three main evacuation sites -- one for each of the resistance groups. The rest are split among six smaller sites.

The biggest settlement, called Site 2, is a camp of Son Sann's group north of the Thai border town of Aranyaprathet and within a couple of miles of the border. It holds more than 132,000 refugees, reputedly making it the second-largest Khmer population center after Phnom Penh. Also living at Site 2 are more than 4,000 Vietnamese "land people" who fled Cambodia overland.

Thai authorities have insisted lately on strict separation between Cambodian guerrillas and the civilian populations at the camps to deny the Vietnamese any pretext for attacking them. In addition, other evacuation sites have been prepared farther inside Thailand, but relief officials worry that any Vietnamese attack may still be disastrous.

"It is estimated that if we want to evacuate Site 2, it will take at least two days to move everyone," one relief official said. Furthermore, he added, "about 40 percent of the population literally need to be carried" because they are children.