Vietnamese troops waging an offensive against Cambodian resistance forces along the Thai-Cambodian border today repulsed attempted counterattacks by anticommunist guerrillas trying to recapture a major base overrun by the Vietnamese yesterday, according to Thai military officials and western diplomatic sources.

The attack on the Rithisen camp, the largest settlement of the Khmer People's National Liberation Front guerrilla group, just across the border from Nong Samet, Thailand, sent more than 60,000 Cambodian civilians fleeing to Thailand. The Vietnamese assault was seen here as part of a military and political offensive aimed at breaking up and discrediting a three-party resistance coalition, which boasts U.N. recognition for a government nominally set up in a "liberated zone" along Cambodia's border with Thailand.

In Washington, the State Department said that "Vietnam's continuing aggression in Cambodia, directed chiefly against civilian camps and noncommunist military forces, is contemptible."

The statement, from spokesman Alan Romberg, said: "The Cambodian people, after so many years of war, should be allowed to choose their own government and to live in peace. Unfortunately, there is no sign that Hanoi is prepared to accede to the world community's call for a Vietnamese withdrawal and the reestablishment of Cambodian sovereignty through free elections under international auspices."

The stepped-up Vietnamese campaign coincided with a trip to Vietnam, Cambodia and Laos by two U.S. congressmen, who said they came away "disappointed" by the prospects for a Cambodian settlement and for a proposed deal to send Vietnamese reeducation camp inmates to the United States.

In a news conference here tonight, Reps. Stephen Solarz (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asian and Pacific affairs, and Robert Torricelli (D-N.J.), a member of that committee, also expressed disappointment at apparently having been snubbed by senior Thai officials for visiting the Cambodian capital, Phnom Penh, where a Vietnamese-installed government holds power.

Solarz said he detected some "movement" in the Vietnamese position on Cambodia compared to a visit he made four years ago. But he said the refusal of Vietnamese leaders and their Cambodian proteges to indicate the kind of political solution they might accept suggested that they were more interested in dividing the resistance than in seeking "a realistic settlement of the conflict."

Solarz and Torricelli said they met Vietnamese Premier Pham Van Dong and Foreign Minister Nguyen Co Thach, as well as the foreign ministers of Cambodia and Laos, during a four-day trip to the three countries.

In Hanoi's latest dry-season offensive against the Cambodian resistance, Vietnamese troops backed by tanks and armored personnel carriers overran most of the Rithisen camp yesterday and set fire to its bamboo and thatch huts.

It was not immediately possible to confirm refugee reports that at least 100 Cambodians were killed by heavy shelling in the Vietnamese assault. Western relief officials said only that more than 50 wounded Cambodians were evacuated for treatment at hospitals in Thailand.

As guerrillas loyal to Son Sann, prime minister of the resistance coalition, battled in vain to recapture the camp today, more than 25,000 Cambodian civilians at his group's base in Ampil, about 18 miles north, braced for a Vietnamese assault amid sporadic shelling, Thai military sources reported.

About 100,000 of the estimated 250,000 Cambodian refugees living in resistance camps along the border have been forced to flee to Thai territory since the Vietnamese opened their dry-season offensive earlier than expected last month. So far, the offensive seems directed mainly at the two noncommunist groups in the resistance coalition -- Son Sann's Khmer Front and a smaller faction loyal to Prince Norodom Sihanouk, the former chief of state -- although the third party, the communist Khmer Rouge, is by far the biggest military threat to the Vietnamese.

Solarz and Torricelli said the Vietnamese and Cambodian officials they met remained adamant in their refusal to negotiate with the Khmer Rouge, who ruled Cambodia brutally for nearly four years until ousted by a Vietnamese invasion launched six years ago yesterday.

Cambodian Foreign Minister Hun Sen expressed willingness to negotiate with the Son Sann and Sihanouk groups, but only if they repudiated all links with the Khmer Rouge, Solarz said. He said this was one example of movement, but that it did not go far enough to foster negotiations.

Solarz said he came away convinced that "the only way the Cambodian problem can be solved is at the negotiating table, not on the battlefield," and that it was worthwhile to explore every possibility for talks.

"We were disappointed by our discussions in Hanoi and Phnom Penh," he said, "but that doesn't mean the effort was not worth making."

The two congressmen said they were especially discouraged by what they saw as a Vietnamese retreat on a longstanding offer to send an estimated 8,000 to 10,000 reeducation camp inmates to the United States following U.S. moves to take Hanoi up on the idea.

The congressmen said the Vietnamese told them the inmates would be released only if the United States made commitments that they would not engage in any opposition activities against the Hanoi government. The Vietnamese claimed the United States was backing efforts by exiles to destabilize the government and wanted "sweeping assurances" that Washington was constitutionally unable to give, Solarz said.