Vietnam charged yesterday that Cambodian forces had launched an infantry and naval attack around the southwestern coastal town of Hatien close to their disputed border.

The Voice of Vietnam called the assault a "grave escalation" in the violent border conflict that began shortly after Communist troops conquered U.S.-backed governments in Saigon and Phonm Penh in April 1975.

Vietnam's charge follows reports last month that Hanoi was making an intensive effort to defuse the conflict by withdrawing a force of several thousand that it sent into Cambodia last fall and clearing the border area of military units to a depth of several miles. Since then, however, both sides have accused the other of making savage forays into the frontier area.

The Hanoi broadcast gave this account of the fighting near Hatien:

Striking before dawn on Tuesday, the Cambodians sent three battalions across the marshy ground north of the city. By nightfall two battalions occupied the villages of Xaxia and Kylo four miles north of the city and the third drove as far as Diatang, 1 1/2 miles to the north. Heavy fighting was reported to be still going on there.

Simultaneous with the drive across the frontier area, Cambodian naval vessels put ashore a battalion-sized force that was said to be advancing on Hatien from the west and northwest.

Hatien, a fishing port, has lost more than 90 per cent of its former population of 30,000 since the border clashes began, according to Western journalists who visited the area earlier this year.

When the Vietnamese crossed into Cambodian last fall Western observers interpreted it as a move to force Phnom Penh to the negotiating table.

On Dec. 31 Cambodian broke relations with Hanoi and said it would not negotiate until the Vietnamese units were withdrawn and Vietnam promised to abandon what Phnom Penh said was an attempt to exercise political domination of the Indochina peninsula.

The enmity between the Vietnamese and Cambodians goes back several centuries. At one time Cambodians controlled most of what is now the southern part of Vietnam but were driven away by the Vietnamese.

Meanwhile, Yugoslavia's official Tanjug news agency reported from Phnom Penh yesterday that officials there say they are prepared to negotiate a solution to the border conflict.

Quoting "responsible quarters," the Tanjug report said that issues preventing good and friendly relations between the two countries will disappear once the government and Communist Party of Vietnam agree to respect the national independence, sovereignty and territorial integrity of Cambodian and its right to decide about its future alone.