Ieng Sary, the number two official of the deposed government in Cambodia, has sent a message with refugees crossing the Thai border saying that he would like to emerge from hiding and may seek Thailand's assistance in arranging an escape route.

The Thai government confirmed that it had received a letter from Ieng Sary, the deputy premier and foreign minister of the overthrown government, early today at a town on the Thai-Cambodian border.

The government refused to disclose contents of the letter. Bangkok newspapers reported that the deputy premier had sought political asylum in Thailand but that could not be confirmed. There was speculation that Ieng Sary merely wanted to arrange passage through this country to a safe haven, perhaps China.

Ieng Sary is the highest member of the fallen government to be heard from since the Cambodian capital, Phnom Penh, was overrun early this week by force supported by Vietnam.

The fate of Pol Pot, the Cambodian prime minister, is still unknown. He is thought by some observers to be hiding in a mountainous region of western Cambodia, hoping to regroup enough forces to fight a protracted guerrilla war for control of the country. But there is no evidence that Pol Pot is still alive, and there have been unconfirmed reports that he was killed shortly after the fall of Phnom Penh.

Ieng Sary is the best-known spokesman for the deposed government that had controlled the country sincd 1975. As its foreign minister he almost alone had sought to explain its radical domestic politics to the world.

His surfacing could present a problem for the Thai government, which has tried to remain uninvolved and which does not want to become a haven for the defeated Cambodian rulers.

His letter was passed to a Thai border official early today near the town of Arayaprathet by at least three Cambodians who emerged briefly and then disappeared. It was transmitted later to officials at the Foreign Ministry in Bankgok.

Another unconfirmed report said Ieng Sary and deposed president Khieu Samphan had arrived at the Cambodian border town of Poipet.

Meanwhile, the Vietnam-supported forces continued to expand their control inside Cambodia. Reports were received here of fighting in the neghborhood of Siem Reap, about 110 miles from the Thai border.

Siem Reap is near the ancient temple complex of Angkor, the symbol of Cambodian nationalism. If the Vietnamese-backed insurgents take Siem Peap, that would clear the way to Battambang, the last provincial capital not yet clearly in their hands.

The rebel movement, called the Kampuchean (Cambodian) National United Front For National Salvation, is now believed to control about three-fourths of Cambodia. Vietnamese air strikes, reported at about 100 a day yesterday, had lessened, according to Western sources.

The Front has pledged publicly that it will not take harsh measures against the former government's troops if they surrender, but it has said in radio broadcasts that die-hard government supporters would undergo "reeducation" when captured.

So far, no large units of Cambodian troops have attempted to flee into Thailand although individual soldiers have been picked up on the Thai side of the long border.

Thailand hopes to avoid any involvement in Cambodia's internal war and government officials have said that refugees fleeing the current fighting would not be accepted. Thailand already is host to more than 100,000 Indochinese refugees, many of them Cambodians who fled when the Khmer Rouge forces took control in 1975.

News services reported these developments:

The insurgents' provisional government, headed by a dissident Communist Party member named Heng Samrin, has been recognized by Vietnam, Laos, Afghanistan and all the European Soviet Bloc nations except Romania.

The Romanian Communist Party newspaper Scintea denounced the military takeover as a "heavy blow for the prestige of socialism" and a threat to East-West detente.

"Nothing can justify support given to elements rising against their own government," Scintea wrote.

China called the formation of a Cambodian Revolutionary Council to run the country a "puppet show" staged by Moscow and Vietnam to disguise Vietnam's aggression in Cambodia.

The official New China News Agency, monitored in Hong Kong, was commenting for the first time on the rebels' provisional government. It described the Council's formation as the "latest move on the part of the hegemonists in Moscow and Hanoi to hoodwink the world."

Rep. Lester Wolff (D-N.Y.), leader of a seven-member congressional delegation, canceled a trip from Bangkok to Hanoi, saying Vietnam's invasion made the visit "inappropriate... at this time."

The five-member Association of Southeast Asian Nations said it would meet in Bangkok Friday to discuss Cambodia. Philippine Foregin Minister Carlos Romulo said in a statement that the war in Cambodia has the potential of "unleashing a chain of events with grave consequences for all of Asia."