Former Cambodian head of state Prince Norodom Sihanouk said early today that he was ready to join without preconditions a united front of resistance groups opposing the Vietnamese occupation of Cambodia and he held out the possibility that this could lead to a "compromise" with Hanoi.

Sihanouk made the statements to reporters after arriving here late yesterday from Paris to join two other Cambodian resistance leaders in unprecedented talks on the possibility of forming such a united front. Although Sihanouk, his former prime minister Son Sann and Khmer Rouge President Khieu Samphan have met bilaterally before, the Singapore gathering marks the first time they all have come together to thrash out a common position.

A union of the three groups has been promoted by Vietnam's noncommunist neighbors as well as China and the United States as a way to increase pressure on Hanoi to withdraw from Cambodia.

Sihanouk, apparently dropping an earlier demand that he be named president and prime minister of any coalition government with the Khmer Rouge, said "there is no condition from Sihanouk" on joining a front. He also said he was "ready to support any agreement accepted by both the Khmer Rouge and Son Sann," who heads the noncommunist Khmer People's National Liberation Front.

Sihanouk said, however, that he doubted that Khieu Samphan and Son Sann could reach agreement on the terms for forming some kind of union. Both men arrived earlier yesterday from Bangkok.

A further aim of a united front would be to improve the international standing of the Khmer Rouge government that was ousted by Vietnamese invasion troops in January 1979 but which still holds Cambodia's seat at the United Nations.

The U.N. General Assembly is due to vote later this month on the credentials of the Khmer Rouge, which has been blamed for the deaths of more than 1 million Cambodians during its destructive four-year rule.

Speaking to reporters after arriving at his hotel here, Sihanouk said that although Cambodians still in the country "don't like the Vietnamese and feel humiliated" by losing their independence, "they would prefer the status quo" to the return of the Khmer Rouge to power.

"I am in a terrible dilemma because I know my people in Cambodia don't want me to participate in a united front with the Khmer Rouge," Sihanouk said. But at the same time, he added, Cambodians in exile were urging him to fight the Vietnamese occupation.

"To obey my people inside Cambodia or outside Cambodia, that is the question," he said. "It is very Shakespearean."

Sihanouk justified his decision to join a united front with the Khmer Rouge and Son Sann on the grounds that "I know the Vietnamese will never let the Khmer Rouge take power again as in 1975."

The Vietnamese, he said, were "the guarantee to prevent the Khmer Rouge from being the masters of the Cambodian people."

Thus, he told reporters, "we could at maximum have a compromise with Vietnam."

In a written statement issued before he left Paris, however, the 59-year-old prince expressed pessimism about the prospect of forcing the Vietnamese to withdraw from Cambodia.

He said a solution did not seem possible unless Vietnam were defeated militarily and no other country appeared willing to battle Hanoi on behalf of the people of occupied Cambodia.

Nevertheless, Sihanouk said, he was willing to join a united front to try to "find a way out of this apparently irreversible situation . . ."

In a separate statement, Son Sann repeated several tough conditions for joining a coalition with the Khmer Rouge.

The 70-year-old leader demanded foreign aid to allow him to increase his forces by "30,000 armed men" to put them on a par with the Khmer Rouge troops.

He also insisted that his group take over the leadership of any coalition and hold most of its ministerial portfolios, and that "certain most compromised leaders" of the Khmer Rouge go into exile. He named former premier Pol Pot, who still heads the military and the Communist Party, foreign minister Ieng Sary, defense minister Son Sen and Army commander Ta Mok.

Sihanouk said he doubted the Khmer Rouge would accept such conditions.

Because of these differences, Singapore has been playing down the prospect that any actual union will come out of the Cambodian leaders' meetings here. Instead, diplomats said, Singapore and other members of the Association of Southeast Asian Nations (ASEAN), the tacit sponsor of the conference, hope it will result in a joint public statement opposing the Vietnamese occupation and expressing willingness to continue working toward a coalition.