After months of delays, international aid organizations and the Hanoi-backed government of Heng Samrin in Phnom Penh reached an initial agreement this week to begin a $100 million emergency relief program to save the famished people of Cambodia.

Informed sources here said the Heng Samrin government finally accepted the minimum requirements for such a massive relief effort.

The acceptance marked the first time that Cambodian authorities have agreed to cooperate with international bodies since Communist insurgents took control of the country in April 1975.

At that time, Khmer Rouge rebels forced the surrender of the U.S. backed government of President Lon Nol and replaced it with a radical administration led by guerrilla leader Pol Pot as prime minister. His government was overthrown this year by a Vietnamese invasion force. Heng Sam rin was then installed as head of state.

President Heng Samrin arrived in Hanoi Wednesday at the head of a high-level Cambodian delegation, the Vietnam News Agency said. The agency said Vietnamese leaders and "hundreds of thousands of Hanoians" turned out to welcome Heng Samrin.

It was Heng Samrin's first official visit to Vietnam since he took power Jan. 7, news services reported. An estimated 170,000 Vietnamese troops remain in Cambodia, propping up the government and fighting the guerrilla forces of Pol Pot.

The emergency relief agreement is vital for the survival of tens thousands of Cambodian civilians, informed sources here said.

"Nothing approaches the desolation" in Cambodia, said a Western official who recently returned from Phnom Penh. "The disease, starvation, orphans . . . it is all incomprehensible. I have seen nothing like it before, not Bangladesh, not anything."

The breakthrough came during the second round of negotiations between the International Red Cross, the U.N. Children's Fund (UNICEF) and the Cambodians. The aid representatives won approval of their general relief plan, direct flights from Bangkok to Phnom Penh and the installation of sophisticated communications with the outside world. Most importantly, they gained acceptance of plans to "move towards" establishing a permanent aid headquarters in Phnom Penh next month, according to aid sources.

Without such oversight, the Red Cross and UNICEF could not insure that food would be distributed to civilians rather than to soldiers in the midst of war.

At a minimum, the estimated 2 million Cambodians living under Heng Samrin will need 108,000 tons of rice, 15,000 tons of sugar, 8,000 tons of butter and large quantities of oil as well as medical supplies and technicians during the next year.

Perhaps another 800,000 Cambodians are under the control of the Pol Pot resistance troops.Under international charters they, too, should receive aid. But Pol Pot has raised more obstacles than Heng Samrin, and that effort may be doomed.

The relief organizations do not have yet the funds to pay for such an enormous program, and officials fear that international fund raising efforts could become bogged down in international squabbles about who legally heads Cambodia, Pol Pot or Heng Samrin.

Meanwhile, relief experts estimate that there is only one birth for every 10 deaths in Cambodia today. They say political instransigence could make that figure worse.

A team of French doctors who recently visited Phnom Penh reported that people were dying of disease that are usually easily curable. Other experts predicted that thousands of Cambodians who survived a decade of intermittent warfare and political upheaval could die from this year's plagues.

Pol Pot is considered the legal representative of Camobdia by most international organizations and countries except those in the Soviet bloc, which includes Vietnam.

After he was overthrown in January, Pol Pot and an estimated 20,000 of his Khmer Rouge soldiers took to the jungles. They have kept the Vietnamese and Heng Samrin's troops from consolidating control of the country, and the fighting has kept the peasants from growing food.

"There are very, very few rice fields cultivated, and you do not see fruit in the city markets," said a Western observer.He said the Phnom Penh orphanage is filled with undernourished children.

"There are 492 children there and all they can eat is rice soup and banana shoots," he said. "None of them are infants or very little children. I think the infants were the first victims in this war."

The potential for epidemics and plagues is so high that aid officials plan to bring a doctor to Phnom Penh next week in hopes that permission will be granted for permanent offices and medical care can begin immediately.

UNICEF and the International Red Cross already have flown in 5 tons of medicine purchased in Ho Chi Minh City (formerly Saigon). They estimate another $10 million worth of medicine will be needed for the rest of the year.

So far only 40 Cambodian doctors with any training have been found in the country.

"Those (Cambodians) who are alive are in very bad shape," said a Western expert who recently returned from Phnom Penh. "Cambodians are strong people, but they have not been able to survive this. The anemia, the listlessness, it is all there. Relief must come quickly."