A top Singaporean official today announced plans to form a unified Cambodian resistance movement, and said Singapore would provide arms to the movement if its three factions can form a coalition government in exile.

Sinnathamby Rajaratnam, Singapore's deputy prime minister in charge of foreign affairs, said at a news conference here that leaders of the communist Khmer Rouge, and two noncommunist groups -- the Khmer People's National Liberation Front and a group led by former Cambodian head of state Prince Norodom Sihanouk -- had accepted a Singaporean unity proposal stating that each faction would retain its identity in an exile government.

Rajaratnam said Singapore would be willing to provide arms aid to the noncommunist elements in the coalition, which would try to overthrow the Vietnamese-backed government that has ruled Cambodia since 1978. He stressed that "other democratic countries" should also contribute if they wanted to see a "non-Khmer Rouge government" replace the current regime in Phnom Penh.

Rajaratnam's remarks and a press statement issued by Singapore's embassy here marked the first time that a member of the noncommunist Association of Southeast Asian Nations (ASEAN) has publicly expressed willingness to aid Cambodian resistance groups militarily.

Up to now, the only significant military aid has come from China, and it has benefited mostly the Khmer Rouge, whose brutal, 3 1/2-year rule of Cambodia from 1975 to 1978 caused most countries to shun contact with it. The noncommunist nations of ASEAN would like to see the Vietnamese out of Cambodia, but have been reluctant to provide weapons to the Khmer Rouge.

Rajaratnam indicated that ASEAN would solicit military aid for the noncommunist Cambodian groups from Western countries, including the United States. A U.S. source in Washington said there are no plans to provide any such assistance.

A Western diplomat here said Rajaratnam's remarks signaled "an obvious turning point," but that it was not yet clear whether Singapore's position was endorsed by the other four members of ASEAN: Indonesia, Malaysia, the Philippines and Thailand.

Explaining his written statement, which calls for a coalition to exert "military pressure" on Vietnam to withdraw its troops from Cambodia, Rajaratnam said the other ASEAN countries have given Singapore "a mandate to pursue a coalition government." Singapore is currently chairman of ASEAN.

The Singaporean's statements coincided with preparations by Son Sann for a fund-raising tour of Western countries.

Adding to the flurry of activity, Sihanouk has summoned his representative in Bangkok, In Tam, to France for consultations.

Although his Moulinaka guerrilla group is the smallest of the three resistance factions, Sihanouk is considered the one indispensible figure in any coalition because of his international standing and his relative popularity in Cambodia. Press reports from France, where Sihanouk now lives, quoted him as saying last week he would not participate in a communist-dominated exile government, but that he had asked his advisers to examine the proposal for a three-way coalition.

In Tam, Son Sann and two Khmer Rouge leaders, Khieu Samphan and Ieng Sary, met individually and jointly over the past two days with Rajaratnam, Singaporean Foreign Minister Suppiah Dhanabalan and Thai Foreign Minister Siddhi Savetsila, according to the Singaporean statement.

It said the Cambodian leaders discussed the work toward a coalition by an ad hoc committee set up after they signed a joint declaration in Singapore in early September calling for unity in the battle against the Vietnamese occupation.

The reluctance of the groups headed by Son Sann and Sihanouk to join a coalition with the Khmer Rouge have been addressed in the agreement announced today by switching the basis of the union from a traditional coalition to a structure giving each group autonomy.

Today's statement said the three groups "accepted a Singapore proposal, supported by Thailand, that the coalition government of Democratic Kampuchea to be established should, at this juncture of the struggle, be one in which each faction would retain its identity and be free to propagate its own distinctive political program and philosophy for the future of Cambodia."

Singapore further proposed that if the Vietnamese withdrew, the coalition would "be automatically dissolved" to pave the way for a new government chosen under U.N.-supervised elections as stipulated in U.N. resolutions on Cambodia.

Rajaratnam stressed in his airport news conference today before leaving Bangkok that since Son Sann and Sihanouk were "not members of any government," it was difficult to aid them "legally."

He added, "but once they are in a government, we can give aid. We can give arms as we do to any government."