Vietnamese forces occupying Cambodia attacked a Cambodian resistance settlement near the border with Thailand today, forcing nearly 20,000 refugees to flee toward the border and raising fears among resistance leaders that the move could herald an early dry-season offensive.

Backed by artillery fire, the Vietnamese troops assaulted the Nong Chan camp of the noncommunist Khmer People's National Liberation Front led by Son Sann, the prime minister of a U.N.-recognized coalition government opposing the nearly six-year-old Vietnamese occupation of Cambodia.

According to a front spokesman in Bangkok, Boun Say, the Vietnamese began their attack at dawn, fought for a few hours and then resumed their assault again in the afternoon. He said the Vietnamese overran some forward positions east of the camp that were manned by the front's guerrillas before communications with Nong Chan were lost. He said about 2,000 Vietnamese troops were involved.

"It has started already," Boun Say said when asked about the prospects of an early Vietnamese offensive in the November-to-May dry season. Thai military officials also have been predicting an early offensive based on reports of Vietnamese troop and equipment movements in recent weeks and an incursion into Thai territory in Surin Province earlier this month.

In the past, the Vietnamese usually have launched annual dry- season offensives against the guerrillas around February, when the dry terrain favors the movement of troops, tanks and supplies. However, this year's rainy season has been relatively mild, and the terrain already may be favorable for an offensive, Thai military officials have said.

According to western relief officials, about 18,000 Cambodian civilians fled to preliminary evacuation sites along the Thai-Cambodian border south of the Thai town of Aranyaprathet after the Vietnamese attack began. Some wounded civilians were evacuated to hospitals inside Thailand, but a complete casualty toll was not available.

Relief officials said they feared that if today's attack does in fact signal the start of an offensive, the fighting could spill over to the bigger Nong Samet camp a few miles to the north with more serious consequences.

The Khmer Rouge, the largest guerrilla group battling the Vietnamese, ruled Cambodia brutally for nearly four years before Vietnam invaded in December 1978 and captured the capital, Phnom Penh, the following month. In 1982, the Khmer Rouge joined an uneasy coalition with the strongly anticommunist Khmer People's National Liberation Front and another noncommunist faction led by Prince Norodom Sihanouk.