Vietnamese forces have withdrawn from a Cambodian resistance base they overran two weeks ago at the Thai-Cambodian border and are moving northward into position for a possible assault on the headquarters of the main noncommunist guerrilla group battling the Vietnamese occupation of Cambodia, according to western and Thai diplomatic and intelligence sources.

The withdrawal last Thursday from a camp run by the Khmer People's National Liberation Front (KPNLF) opposite the Thai border village of Nong Chan came as Cambodian peasants conscripted by the Vietnamese completed a key stretch of a new "strategic barricade" being built inside Cambodia parallel to the volatile border, the sources said.

Noting that the Vietnamese stayed in the Nong Chan camp an unusually long time after storming it Jan. 31 following a fierce tank and artillery barrage, diplomats said one reason for taking the camp might have been to protect workers building the barricade.

The onslaught forced 30,000 Cambodian refugees living at the camp to flee into Thailand, where Thai authorities stopped them for several days opposite an antitank ditch in a battle zone near the border before allowing them to move to a new camp in a safer area.

The attack on the Nong Chan camp illustrated an apparent change in Vietnamese strategy during this November-to-May dry season, western diplomats said. Vietnamese forces have mounted dry season offensives against Cambodian resistance groups along the western border for the last four years since invading Cambodia and toppling the brutal Khmer Rouge government in January 1979.

Unlike last year, the Vietnamese so far have refrained this time from mounting a large-scale sustained offensive. Instead they have carried out selective strikes against resistance targets, using only the forces necessary for the operations. In addition, they have concentrated more attention this year on the weaker, noncommunist resistance groups instead of trying to wipe out the 30,000 to 40,000 hardened Communist guerrillas of the Khmer Rouge.

Now the estimated 3,000 Vietnamese troops that overran the Nong Chan camp have regrouped just east of the site and are moving north toward other Cambodian resistance camps, sources said.

The western and Thai intelligence sources believe a likely target is a camp opposite Ban Sangae containing the military headquarters and some training facilities of the Khmer People's National Liberation Front led by former Cambodian prime minister Son Sann. Other potential targets are a Khmer Rouge camp at Phnom Chat and one to the north run by the faction of former Cambodian head of state Prince Norodom Sihanouk.

Sihanouk and Son Sann hold the posts of president and prime minister respectively in a loose coalition with the Khmer Rouge. Sihanouk chaired a meeting of the coalition in Cambodia when he visited the border area last month before the Vietnamese attack on the Nong Chan camp.

Another possible Vietnamese target is a sprawling KPNLF camp opposite the Thai border village of Nong Samet. About 50,000 Cambodian refugees live there on flat, exposed terrain that is difficult to defend. Living in their midst until recently were 1,890 Vietnamese refugees crammed into a detention camp called NW82.

Thai authorities recently closed the camp after allowing U.S. and other western immigration officials to process the Vietnamese for resettlement abroad. After a slow start in which officials of the U.S. Immigration and Naturalization Service came under criticism for turning down many applicants, including some with close relatives in the United States, the INS reconsidered a number of cases and ended up taking 1,100 refugees, or 82 percent of the applicants.

Other countries took nearly 700 of the "land people," who made a hazardous overland trip across Cambodia to reach the Thai border. The remainder, 95 young single males rejected by all the resettlement countries, were moved to Thailand's Khao-i-Dang refugee camp.

In the future it may be more difficult for both Vietnamese and Cambodian refugees, as well as black-market traders, to reach the Thai border at all.

Western diplomatic sources confirmed that the Vietnamese early this month started building a "strategic barricade" consisting of a dike with a road on the eastern side and a 20-foot-wide, 10-foot-deep, spiked ditch on the western side.

An official of the Vietnamese-installed Cambodian government has described it as an antitank ditch designed to stop both resistance forces and the Thai Army. But diplomats here believe the barricade was built for other reasons as well.

"It's for a variety of purposes, not the least of which is to keep people in," said one western diplomat. He said 60,000 Cambodians conscripted from half a dozen districts were working on the barricade and have completed a stretch of three to four miles near the Nong Chan area.

According to an Asian Wall Street Journal correspondent who visited the Cambodian city of Sisophon recently, a district official said the barricade would run 19 miles from the village of Kop four miles south of Nimit on Highway 5 to a point about five miles north of Yeang Dangkum.

Called the Vat Can Defensive Line in Vietnamese blueprints, the barricade will be built roughly parallel to the Thai border from three to nine miles inside Cambodia, the Journal quoted Sisophon district chairman Lim Chhon as saying.

A western diplomat who closely monitors Cambodian developments said that while the barrier might "choke things off in the Nong Chan and Nong Samet areas," its effectiveness would depend on how well it was patrolled.

Intelligence sources now estimate that there are 160,000 to 180,000 Vietnamese troops in Cambodia, 35,000 of them stretched along the Thai-Cambodian border. The sources say this is not enough to mount the k