Faced with a new anti-Vietnamese coalition that appears to be attracting supporters, Vietnam is bolstering its forces in Cambodia near the border with Thailand, according to Thai and Cambodian sources.

The Vietnamese occupation forces have moved in fresh troops and equipment, these sources said, and stepped up shelling of the border settlements of the Cambodian resistance groups in recent weeks. The moves appear to be possible preparations for a new offensive during the dry fall season, they said.

Nevertheless, the two noncommunist groups in the coalition say they are still awaiting supplies that they expected to receive in return for joining the union with the Chinese-backed Khmer Rouge guerrillas.

Hanoi has dismissed the coalition as nothing more than a pretense of respectability for the ousted Khmer Rouge government, which carried out a reign of terror in Cambodia for nearly four years before being driven from power by invading Vietnamese forces in January 1979.

According to Western diplomats, however, Vietnam was surprised and worried when a coalition government was formally constituted July 9 following a June 22 agreement in Kuala Lumpur, Malaysia. They said Hanoi fears that the participation of the noncommunist groups led by former Cambodian leader Prince Norodom Sihanouk, who became president of the coalition, and Son Sann, who became prime minister, would undercut efforts to gain legitimacy and popular acceptance for the Vietnamese-backed Heng Samrin government in Phnom Penh and further isolate Hanoi internationally.

According to the diplomats, Thai officials and Cambodian sources, a Vietnamese "unilateral partial withdrawal" of forces from Cambodia announced July 7 basically amounted to a rotation of troops in which long-serving individuals from various units were replaced. No units were actually withdrawn, these sources said.

Moreover, according to the sources, the number of Vietnamese soldiers arriving in Cambodia probably exceeds the number being pulled out and they are being reinforced by new supplies.

The assistant director general of the Thai Central Intelligence Department, Thanu Chalarak, was quoted as telling a panel discussion Aug. 20 that the Soviet Union recently supplied more than 100 tanks and some 155-mm artillery pieces to strengthen Vietnamese troops along the border.

A spokeswoman for Son Sann's Khmer People's National Liberation Front said this week that the new Vietnamese troops and equipment had appeared all along the border with Thailand.

"I think they are preparing something for the dry season," she said. The rainy season, in which the Vietnamese find it difficult to maneuver, normally favors the guerrillas and lasts from June to October.

The spokeswoman said the coalition has improved the front's position diplomatically but has made no difference militarily.

"We thought we would get aid quickly" once the coalition was formed, she said. "But it has been two months now, and we're still waiting."

Son Sann, who briefly served as prime minister under Sihanouk in the late 1960s, told reporters during a recent Southeast Asian tour that he looked to China as a main source of military assistance to his group.

With 9,000 armed soldiers, 3,000 others without arms and a civilian population of 155,000 in his group's "liberated zones" in western Cambodia, Son Sann said his main need was not more manpower but weapons, ammunition and money.

The United States publicly continues to insist that it is neither giving nor considering military assistance to the Cambodian guerrillas. However, the question of financial aid that could be used to buy arms elsewhere remains murky.

So far, despite clauses in the coalition pact favoring the Khmer Rouge, the main winner in the arrangement appears to be Sihanouk.

Following a visit to the border area last month by the mercurial prince, about 14,000 Cambodian refugees at the Khao-I-Dang camp in Thailand signed up to join his one-year-old Sihanoukist National Army. Of these, 2,000 to 3,000 already have moved to a camp in northwestern Cambodia at O Smach, now renamed Sihanouk Borei, or "Sihanouk Town." The rest reportedly are awaiting the establishment of adequate facilities for them there.

In addition, 300 to 600 Khmer Rouge soldiers and civilians tried to cross the border to defect to Sihanouk's faction last week, but many were turned back by Thai authorities, diplomats said. A spokesman for the Moulinaka group, the main component of the Sihanoukist National Army, said about 170 Khmer Rouge defectors joined one of the faction's camps a few days ago.

Sihanouk's trips to the border during his July 3-17 stay in Thailand "demonstrated that he still has unique appeal among Cambodians on the ground," one Western diplomat said. Another noted that the former Cambodian monarch's new activism seemed to raise refugees' hopes but that the establishment of the anti-Vietnamese coalition has not translated into any coordination of military activity. Furthermore, the reaction of Cambodians in the interior is not yet clear, and many are still believed to fear a return to power of the Khmer Rouge more than they oppose the Vietnamese.

"People are in a state of expectancy," said the spokeswoman for Son Sann's group. She cited a "wait-and-see" attitude among Cambodians in the interior. For its part, she added, the group is not accepting Khmer Rouge defectors because of suspicions that they might be infiltrators.

With about 30,000 to 40,000 hardened guerrillas, the Khmer Rouge has been doing most of the fighting against the Vietnamese occupation force, which numbers up to 200,000 troops. Sihanouk's group is currently the smallest in the coalition, with fewer than 3,000 fighters.

While the group received a shipment of Chinese arms in April, other requests are still "under consideration" by unnamed "friendly countries," a Sihanouk spokesman said.

Holding the coalition together is a clause that each party "shall retain its own organization, political identity and freedom of action, including the right to receive and dispose of international aid specifically granted it."

The three groups overcame a dispute about who should be foreign minister by agreeing to give the Khmer Rouge leader, Khieu Samphan, the title of vice president in charge of foreign affairs. Formerly president of the Khmer Rouge government known as Democratic Kampuchea, Khieu Samphan now ranks second in the three-man hierarchy.

The rest of the "coalition government of Democratic Kampuchea"--so named because the Khmer Rouge government retains U.N. recognition--consists of four tripartite "coordination committees" nominally in charge of finance, defense and other portfolios.

According to diplomats, the groups led by Sihanouk and Son Sann reluctantly accepted a clause allowing Khmer Rouge representatives abroad to continue in office for six months. In addition, the Khmer Rouge won agreement that the government would revert to its control if the coalition collapses.

The sources said Son Sann, whose group has been running short of supplies, accepted this under strong pressure from the Association of Southeast Asian Nations.