About 1,000 Vietnamese troops, backed by 105 mm howitzers, have penetrated a major Khmer Serei (Free Khmer) stronghold just inside Cambodia, according to Thai military sources.

The attack is the first concerted drive against a Khmer Serei settlement along the border and has driven some 30,000 civilians into Thailand, the sources added.

Analysts said the Vietnamese push might be the start of a campaign to clear a 40-mile stretch of the frontier where about half a million refugees are crowded into primitive camps run by the anticommunist Khmer Serei.

Vietnamese soldiers moved about 100 yards into Thai territory during the operation, which began Monday night, the Thai military sources said. Shells apparently fired by Vietnamese gunners fell on Thai positions near the village of Sanro Changan but caused no casualties. Thai guns fired back.

Some analysts believed the incursion was part of an attempt to encircle about 300 soldiers of the Khmer People's National Liberation Movement, the Khmer Serei group that operates in the area. Members of this movement said the troops broke out of the ring.

Firing was heard from across the border on Thursday, indicating the Khmer Serei were continuing resistance.

Thai military sources said the uprooted civilians will be allowed to go to a new U.N.-sponsored camp about seven miles from the border. Analysts said the Vietnamese attack could speed up Thai efforts to move people from two other camps, run by separate Khmer Serei groups along the border to the south. The Thai army feels the camps would be tempting targets for the Vietnamese and might cause fighting to spill into Thailand.

However, Khmer Serei soldiers at the two camps have forcibly prevented people from leaving. The camps' commanders fear their struggle will be lost if their population base is taken away.

The Khmer People's National Liberation Movement, the group attacked on Monday, is ranked by most analysts as the best disciplined of the various Khmer Serei groups that have emerged as a minor third force in the Cambodian conflict.

It is headed by Son Sann, 68, a French-educated politician who served as prime minister of Cambodia under Prince Norodom Sihanouk. Son Sann's whereabouts following the attack were not known.

Among noncommunist Cambodians, Son Sann is widely respected as an honest and capable leader. He is remembered for returning from exile to Phnom Penh shortly before the Khmer Rouge captured the city in 1975. In an emotional speech, he called on the American-supported marshal Lon Nol to leave the country to allow a political settlement with the Khmer Rouge. Lon Nol refused.

Because of Son Sann's wide appeal among Cambodians, the Vietnamese are reported to have been trying to persuade him to join The Heng Samrin administration in Phnom Penh. Thus analysts were surprised that the Vietnamese attacked Son Sann's group.

The movements' forces of up to 2,000 men and women are commanded by Dien Del, formerly a general in the Lon Nol army. Journalists who knew him then remember him as Lon Nol's most capable commander.

Analysts note that the movement, unlike other Khmer Serei groups, has not claimed it can defeat the Vietnamese on the battlefield. Vietnam is estimated to have 170,000 troops deployed in Cambodia.