The main noncommunist resistance group battling Vietnam's occupation of Cambodia has begun to take the offensive against Vietnamese troops stationed near the border with Thailand, according to Western diplomats and Thai military sources.

The Khmer People's National Liberation Front, whose leader Son Sann currently is visiting the United States, long has been criticized by some supporters for avoiding combat with the Vietnamese. But in the last couple of weeks, the front's forces have attacked and overrun several small Vietnamese positions east of the front's base camps, along the Thai-Cambodian border north of the Thai town of Aranyaprathet, according to the front's claims, which are confirmed by Western and Thai sources.

The offensive actions followed secret arms deliveries from Singapore last month to the front and another noncommunist resistance group led by Prince Norodom Sihanouk, informed sources said. Under pressure from Singapore, Thailand and other members of the Association of Southeast Asian Nations, the groups of Son Sann and Sihanouk joined a loose coalition government in July with the Communist Khmer Rouge, who were driven from power by by Vietnamese invasion of Cambodia in January 1979.

In a bid to spruce up the image of the Khmer Rouge, held responsible for the deaths of 1 million to 2 million Cambodians during nearly four years of murderous and destructive rule, Sihanouk became president of the coalition government and Son Sann its prime minister. The Khmer Rouge leader, Khieu Samphan, became vice president in charge of foreign affairs. The noncommunist groups have said that by joining with the Khmer Rouge, they hope to receive additional overseas support for their efforts to oust the Vietnamese.

Evidently making good that expectation, Singapore has supplied the Khmer People's National Liberation Front with light infantry weapons sufficient to equip about 2,000 troops, sources said. Sihanouk's National Army reportedly has received arms for about 1,000 troops.

Until now, all three groups have been armed almost entirely by China, which has supported the Khmer Rouge for years. With an estimated 30,000 to 40,000 troops, the Khmer Rouge remains the major force opposing the 200,000-strong Vietnamese occupation army in Cambodia. However, Son Sann's group, with about 9,000 men under arms, and Sihanouk's, with an estimated 3,000, have insisted that they could expand if given additional weapons.

In communiques last week, the front claimed to have killed 42 Vietnamese soldiers and wounded more than 50 in operations in western Cambodia, north of Route 5 between Poipet and Sisophon, Sept. 20 to 28. The front said its forces suffered four killed and 27 wounded in the attacks on Vietnamese checkpoints and surveillance posts. Western diplomats said that the front's claims of Vietnamese casualties may have been exaggerated, but noted that lists of captured equipment were fairly modest.

According to one well-informed diplomat, the front had sent a column east from its camp at Nong Chan to remove some Vietnamese checkpoints. The attacks were so successful, he said, that the organization expanded the operation and overran a company-sized position near Yeang Dangkum, about four miles east of Nong Chan. About 2,000 troops were involved in what amounted to the front's largest operation since it was founded three years ago, the diplomat said.

"They surprised the Vietnamese," he said. "They tested them, found they gave and followed up." The diplomat added that Hanoi's units, consisting of a mixture of veterans and new recruits, "didn't react very well" and continued to be in such disarray this week that Cambodian traders and refugees were easily getting through Vietnamese lines to the border without having to pay the usual bribes at checkpoints.

According to a spokesman for the Khmer People's National Liberation Front, arrivals at the front's camps last week included 10 Cambodian defectors from the Army of the Vietnamese-installed Heng Samrin government in Phnom Penh, who brought their weapons with them. The spokesman denied recent Vietnamese claims that hundreds of the front's troops had deserted to Heng Samrin's forces, saying only a former colonel had gone over and that was because of a family problem.

Diplomats see the reported surge in the front's military activity partly as aimed at impressing the group's actual and potential military backers.

"The KPNLF has been under some criticism for failure to show military muscle," one diplomat noted. In particular, China has denigrated the front's military leadership and fighting ability, apparently withholding further arms deliveries despite the formation of the coalition government.

The front's attacks come amid a continuing Vietnamese buildup near the Thai border that many observers believe will lead to a major dry season offensive by Hanoi's forces in December or January. These observers also speculate that this time the front's activities may bring on a major assault by the Vietnamese, who until now have directed most of their efforts at the Khmer Rouge.

In a dispatch last week, the Cambodian government's official news agency in Phnom Penh quoted Foreign Minister Hun Sen as warning that troops near the Thai-Cambodian border might "counterattack" into Thailand if what he called "Thai provocations" were not stopped. Vietnam repeatedly has accused Thailand of supporting the Cambodian guerrilla groups along the border.

"Patience has its limits," Hun Sen was quoted as saying. "If the Thai authorities don't stop, we will have to take up our right to legitimate self-defense."

The remarks were seen here as essentially a threat from Hanoi to send Vietnamese troops across the border, as it did in June 1980.

That the Foreign Ministry in Phnom Penh is completely under the sway of Hanoi would appear to be borne out by the account of a Cambodian defector who formerly served as a senior aide to Hun Sen. The defector, Dy Lamthol, told reporters at Thailand's Khao-i-Dang refugee camp last week that 17 Vietnamese advisers effectively run the Foreign Ministry, issuing instructions from Hanoi to Hun Sen and other senior officials.