Vietnamese forces backed by mortars crossed the Thai border from Cambodia early this morning and occupied two large camps housing Cambodian refugees and two Thai villages, according to reports reaching Bangkok.

While accounts of the fighting conflicted, diplomats said it was Hanoi's most serious incursion into Thailand since Vietnamese forces invaded Cambodia late in 1978, toppling the government of Pol Pot and installing the Heng Samrin government.

Thai military sources reported five Thai soldiers killed and three wounded in the fighting. Thai helicopter crews reported seeing "many" dead and wounded Vietnamese. About 30 wounded civilians were taken to a hospital seven miles inside Thailand.

But another report said that more than 30 Thai soldiers had been killed and 100 wounded, the Associated Press said Western relief officials operating along the border said that general confusion made accurate casualty estimates difficult, especially among the Cambodian refugees.

[Some reports said that the Vietnamese used tanks in their attack on the border camps and that as many as 100,000 Cambodian refugees were forced to flee, news, agencies reported.]

[In Aranyaprathet, Thailand, the AP said that reporters near the scene of the fight said both sides used howitzers.]

Western diplomats said that the two villages and two camps still appeared to be in Vietnamese hands in the late afternoon. Hanoi's forces advanced into a third village two miles inside Thailand before falling back to the other two, according to reports reaching here.

The Thai government, however, maintained that Vietnamese troops had not actually captured the villages and that Thai forces had used artillery and helicopter gunships to drive the Vietnamese back into Cambodia.

[In an apparent effort to play down the incidents, Thai Prime Minister Prem Tinsulononda said that the Vietnamese may have been in "hot pursuit" of the anticommunist Khmer Serei ("Free Khmer") guerrillas in the border region, the AP reported.]

After the Vietnamese invasion in late 1978, hundreds of thousands of Cambodians loyal to Pol Pot's Khmer Rouge or the Khmer Serei fled to the Thai-Cambodian border region to wage guerrilla war against the Vietnamese-backed Heng Samrin government.

About 400,000 Cambodians refugees are estimated to be living in the border region -- about 150,000 of them in U.N.-sponsored camps and the rest in border settlements under Khmer Rouge Control.

Last week, Thailand and the United Nations began a voluntary repatriation program, sending more than 6,000 Cambodians back to their country. This repatriation proceeded over the protests of Phnom Penh and Vietnam, which denounced it as a plot to strengthen Cambodian resistance groups operating along the border.

However, most of those repatriated in recent days have reentered Cambodia from areas far south of where today's attacks occurred. These repatriation points are controlled by the Khmer Rouge.

One Western diplomat suggested that the attack was intended as a warning to foreign ministers of the Association of Southeast Asian Nations (ASEAN,) who are meeting in Malaysia this weekend. Secretary of State Edmund Muskie will also attend the meeting.

ASEAN's five Western-oriented member states -- Thailand, Philippines, Malaysia, Indonesia and Singapore -- have taken a hard line against Vietnam's client government in Phnom Penh and continue to recognize the Khmer Rouge. Vietnam continually has accused Thailand of cooperating with China to supply the Khmer Rouge army and other anti-Vietnamese groups. w

In Washington, State Department spokesman Thomas Reston labeled the Vietnamese action "an aggression" and reaffirmed the United States" security pledge to Thailand.

["We are concerned about reports of Vietnamese aggression in the area," Reston said. "I would like to reaffirm our long-standing commitment to the security of Thailand under the Manila Pact of 1954 and to pledge our continuing consultations in that respect."]

Western diplomats said a food station run by relief agencies outside the Thai Village of Nong Chan, where some 60,000 Cambodian civilians were camped waiting for rice to take back into their country, was one of the camps overrun.

Diplomats also reported fighting had erupted in the area near the Thai border village of Sangae, about 20 miles to the northeast.

Vietnamese patrols almost routinely have intruded into Thai territory in the last year. Most, however, have retreated in the face of warning shells and have not directly engaged Thai forces. Today's incidents appear to be the first time the Vietnamese have taken a Thai Population center.

Since last fall, relief agencies have feared Vietnamese troops might advance on the refugee camps, located is disputed areas of the frontier and housing a collection of Khmer Serei guerrillas.

However, Hanoi's troops have held back, allowing an enormous traffic in oxcarts and refugees to cross to and from the border camps. This "hunman land bridge" has been credited with getting relief supplies to many villages in western Cambodia.

[Western relief officials said today that the Vietnamese attack had halted the "land bridge" aid program, the AP reported.]