Vietnamese forces backed by tanks and artillery launched a major attack today against Cambodian settlements controlled by communist Khmer Rouge guerrillas along the Thai-Cambodian border, overrunning a key base and causing as many as 25,000 refugees to flee into Thailand, according to Thai military and western relief officials.

The attack, which began before dawn and continued all day, killed at least 33 Cambodians and wounded 150, about 70 of them seriously, military sources said.

Following an artillery barrage, Vietnamese troops overran a major Khmer Rouge base at Phnom Chat and captured two smaller camps nearby, according to Thai military reports from the border.

Thai authorities said several shells landed on Thai territory, and the chief of the National Security Council, Squadron Leader Prasong Soonsiri, warned that Thailand reserved the right to "defend our independence and sovereignty."

He said that with time running out in Hanoi's current dry-season offensive, further Vietnamese attacks were expected against more heavily populated settlements controlled by anticommunist resistance groups and that as many as 100,000 refugees eventually might be forced to flee into Thailand.

In Washington, a State Department spokesman said the United States was "appalled" by the "indiscriminate" attacks, and added, "We strongly condemn these attacks."

Thai military authorities said about 6,000 Vietnamese troops were deployed in the area of the besieged camps and were armed with Soviet T54 tanks, U.S.-built armored personnel carriers, new Soviet long-range 130 mm artillery and multiple rocket launchers.

The Vietnamese opened fire with artillery during the night on the Khmer Rouge camp of Phnom Chat, inhabited by about 15,000 Cambodians, and satellite camps holding about 8,000 others at Chang Kakor and opposite the Thai border village of Ban Koktaharn, Thai authorities said. Initially about 15,000 people fled into Thailand, gathering along an antitank ditch just inside Thai territory. Authorities said they would be sent back across the border when the situation improves.

Fighting was reported continuing this evening in the area of the camps, which were reportedly abandoned by the Khmer Rouge as they split into small guerrilla groups.

Foreign relief agencies early today ordered their workers out of the Cambodian settlements in the area north of Aranyaprathet, including the large Nong Samet camp run by the anticommunist Khmer People's National Liberation Front. By this afternoon, a senior United Nations official said, up to 25,000 Cambodians had crossed into Thailand as far as the antitank ditch.

The front's leader, Son Sann, last week reported a buildup of Vietnamese troops in the area and predicted an "imminent attack" on his group.

In an interview yesterday, one of Son Sann's top aides said the Vietnamese started shelling Khmer Rouge positions near Phnom Chat Monday morning and seemed to be getting into position to move north against the front's camp opposite the Thai village of Ban Sangae and south against its camp across the border from Nong Samet. The Ban Sangae camp contains the front's military headquarters as well as about 20,000 civilians. The Nong Samet camp is inhabited by more than 70,000 Cambodian civilians and defended by a few thousand of the Son Sann group's estimated 9,000 to 12,000 fighters.

The aide said the Khmer Rouge had started "retaliating" and that there was "no indication which way the Vietnamese would go from there." She added that "anyway, we are getting ready for any event."

According to Prasong, the Nong Samet and Ban Sangae camps are expected to be the next targets of the Vietnamese, who invaded Cambodia four years ago and drove out the brutal Khmer Rouge regime. The Khmer Rouge, Son Sann's front and another resistance group led by Prince Norodom Sihanouk last year formed an uneasy coalition government that exists largely on paper but holds United Nations recognition as Cambodia's legal administration.

Prasong said he expected the fighting to escalate as the end of the current dry season approaches next month. The May-to-September rainy season hampers movements of Vietnamese armor, troops and supplies, favoring the estimated 30,000 hardened Khmer Rouge guerrillas battling the Vietnamese occupation.

Currently arrayed along the border, Prasong said, are five Vietnamese divisions spearheaded by the crack 5th Division from north Vietnam. He said these troops numbered about 46,000 out of the total Vietnamese occupation force of 160,000 to 180,000.

In a news conference, the Thai national security chief said the Soviet Union recently has supplied the Vietnamese in Cambodia with new weapons including T55 tanks, multiple rocket launchers and antiaircraft batteries. The Cambodian resistance groups have no aircraft, but Thai planes routinely patrol the border area.

Prasong said that between October 1982 and February 1983, at least 18 ships delivered Soviet military supplies and weapons to the Vietnamese at the Cambodian port of Kompong. He said these deliveries were four times greater than in 1981.

Prasong said the Vietnamese have been building up their forces near the Thai border for several weeks since overrunning a camp controlled by Son Sann's front opposite the Thai border village of Nong Chan in early February. About 30,000 Cambodian refugees fled into Thailand after that attack.