With the U.N. General Assembly due to convene next month, noncommunist Southeast Asian nations are stepping up pressures for unity among Cambodian resistance forces opposing the Vietnamese occupation of Cambodia.

Atg stake is Cambodia's U.N. seat, currently held by the ousted Khmer Rouge government but being sought by the protege administration that Vietnam installed when its forces invaded the country 2 1/2 years ago.

While the Khmer Rouge government known as Democratic Kampuchea seems likely to retain the seat in next month's U.N. vote, members of the Association of Southeast Asian Nations are showing signs of concern that the U.N. voting margin for the Khmer Rouge could dwindle and international recognition begin to erode unless a coalition is formed to spruce up its image.

To this end, some Southeast Asian countries apparently have been placing hopes on a prospective conference in Singapore next week during the scheduled visit of the former Cambodian head of state, Prince Norodom Sihanouk.

While Khmer Rouge leader Khieu Samphan was reportedly prepared to fly to Singapore for talks with Sihanouk, a third key resistance leader, former Cambodian Premier Son Sann, has balked at joining such a conference and appears to be increasingly alienating his ASEAN supporters.

In a news conference yesterday at his jungle camp on the Thai-Cambodian border, Son Sann said he would attend a conference with Sihanouk and Khieu Samphan only if three tough conditions, already rejected by the Khmer Rouge, were met first.

Son Sann's conditions are the exile of Khmer Rouge leaders responsible for the brutality of the party's 1975-79 rule in Cambodia, overall control of any coalition government with the Khmer Rouge, and foreign aid to bring his forces up to the strength of the Khmer Rouge Army. Currently, the Khmer Rouge has 30,000 to 40,000 troops, while Son Sann's Khmer People's National Liberation Front counts 5,000 to 6,000 fighters.

The stance of the 72-year-old former premier drew hints from Thailand that it might stop supporting him as a candidate to head the proposed united front against the Vietnamese occupation and might seek a new leader unless he dropped his tough conditions.

In addition, Malaysian Foreign Minister Tan Sri Ghazali Shafie has sent a message to Son Sann asking him to stop obstructing the creation of a coalition in view of the approaching U.N. vote on the Cambodian seat. Specifically, Ghazali Shafie criticized Son Sann for demanding all key ministerial portfolios as a condition for joining the proposed coalition government.

"There is no point talking about a government if Kampuchea does not have a seat in the United Nations," the Malaysian foreign minister said last night in Kuala Lumpur on his return from a visit to Thailand.

Diplomatic sources said the standoff could have serious consequences for the anti-Vietnamese resistance, especially if ASEAN decided to promote another noncommunist leader in place of Son Sann.

"Son Sann has built a name for himself in the United States and Europe, and to a fairly extensive degree inside Cambodia," a Western diplomat said. "If he were suddenly to disappear and someone else take his place, the whole credibility of the movement would be damaged."

"I think he is alienating a lot of people," said an ASEAN diplomat. "There is a certain amount of disappointment" with Son Sann, he added. Son Sann's position apparently is based on a perception that any association with Democratic Kampuchea as currently constituted would discredit him in the eyes of most Cambodians, who generally fear the return of the Khmer Rouge more than they resent the Vietnamese occupation. The ASEAN countries, on the other hand, stress the importance of keeping pressure on the Vietnamese in Cambodia, where the Khmer Rouge provide the only effective military resistance.

Although the Khmer Rouge have failed to attract much signficant political support in Cambodia, they have scored military successes in recent months and have little reason now to accept the sort of terms put forward by Son Sann, Western diplomats said.

Instead, the Khmer Rouge hae been promoting a five-point "common minimum political program" to unite Cambodian groups against the Vietnamese. The program calls for the "various national forces" opposing the Vietnamese to "keep their own organizations with their political individuality as well as their freedom of action."

For his part, the mercurial Prince Sihanouk indicated recently that he wanted to be head of state and possibly premier of a coalition government under the banner of Democratic Kampuchea.

Acknowledging that the proposed union was stymied for the time being, a representative of Sihanouck, In Tam, told a Bangkok newspaper Monday, "It is a complicated issue because everybody wants to be number one in the coalition government."

As a result, diplomats here said, ASEAN now may try to get agreement among the three parties on some kind of joint political statement condemning the Vietnamese occupation before the U.N. General Assembly meets Sept. 15. The vote on the U.N. seat is likely to come a week or two after that, diplomats said.

Meanwhile, the diplomats and Thai officials emphasize that the different parties continue to have contacts and that agreement is still possible.

"People are still talking, and we're hopeful something will break," said a Thai Foreign Ministry official. "We hope we will have something to report to the world before Sept. 15."