Southeast Asia's noncommunist nations, backed by the United States, are launching a new diplomatic offensive to convene an international conference aimed at ending the Vietnamese occupation of Cambodia.

The five-member Association of Southeast Asian Nations (ASEAN) is now "united" in its appeal for an international conference under United Nations auspices even if Vietnam and its main ally, the Soviet Union, refuse to attend, according to Thailand's foreign minister, Siddhi Savetsila.

Siddhi, currently visiting the United States, conferred yesterday with U.N. Secretry General Kurt Waldheim on ASEAN's desire for a conference. The subject was also on the agenda of a meeting in New York yesterday between Waldheim and Vice President George Bush.

State Department officials said U.S. support for such a conference is one of the issues to be raised when Waldheim visits Washington Thursday for meetings with President Reagan and Secretary of State Alexander M. Haig Jr. Waldheim is scheduled to visit Moscow from May 4 to 7, and both ASEAN and Washington want him to urge Soviet participation in the conference.

The U.N. General Assembly called on Waldheim last October to organize a conference on Cambodia in early 1981, but he has been reluctant to do so in the face of Soviet and Vietnamese opposition. Hanoi has also rejected a U.N. call for the withdrawal of all "foreign troops" from Cambodia followed by free U.N.-supervised elections.

The ASEAN countries and the United States see the conference as a forum in which to bring international pressure on the Vietnamese to withdraw from Cambodia and to promote a restructured Cambodian leadership as an alternative to the Khmer Rouge regime of ousted dictator Pol Pot. A key to this effort is the exiled former Cambodian head of state, Prince Norodom Sihanouk, who so far has parried efforts to enlist him in a "united front" with the Khmer Rouge before a U.N. conference is convened.

The five ASEAN countries -- Thailand, Indonesia, Malaysia, Singapore and the Philippines -- are insisting that the conference be held before the next General Assembly session in September, when the controversial issue of Cambodia's U.N. seat comes up for a vote. The United Nations currently recognizes Pol Pot's Khmer Rouge as the country's official representative. The Vietnamese, who drove Pol Pot from power shortly after they invaded Cambodia in December 1978, want U.N. recognition for the government of their protege, Heng Samrin.

According to Siddhi, a retired air marshal, Vietnam may eventually be obliged to seek a way out of its Cambodian quagmire because of a worsening economy at home and declining morale among its troops.