Horrified by reports that at least 2.5 million Cambodians are on the brink of starvation, the International Committee of the Red Cross and the United Nations Children's Fund (UNICEF) are pressing ahead with a large relief effort despite uncertainties about cooperation from the government in Phnom Penh, according to officials here.

Red Cross and UNICEF representatives in Phnom Penh surveyed the area in a 60-mile radius of the city and told their offices in Geneva that very few children are left under the age of 5, that almost no pregnant women are left and that 650,000 children between the ages of 5 and 9 are severely ill from malnutrition.

"Our reports are that many fields are just lying fallow," said a UNICEF official. "In some regions, only 10 to 20 percent of the land has been cultivated."

In addition, crop destruction techniques were used by the ousted government of Pol Pot and the invading Vietnamese in December 1978, leaving no crops in some areas of Cambodia.

The Red Cross and UNICEF also report a scarcity of farming tools and fish nets. The joint relief effort thus will include $4 million for fish nets and tools to help Cambodians overcome a spreading famine.

There are still houses in the countryside, but UNICEF says Pol Pot's Khmer Rouge forces destroyed many villages. As a result, shelter material such as tin sheets will be sent to enable people to build new houses.

Roads are reported in bad condition, and some will have to be rebuilt entirely, officials said. This has made travel around the country very difficult, but even worse is the lack of cars and trucks. That is one reason that UNICEF and Red Cross representatives had a hard time visiting areas further than 60 miles from the capital. The shortage and continuing fighting between government troops and Pol Pot's guerrillas in some parts of the country are hampering distribution of relief supplies.

Already, six planeloads of 150 tons of supplies have arrived. They were unloaded by men who carried the goods off to whatever small transport they could find, because there were no forklifts at the airport and no trucks.

"Almost all these supplies were delivered to people immediately," said one Red Cross official in Geneva. "But we've got to get some machines and trucks in there to do the work. We can't depend on men's backs."

One plane that flew into Bangkok with nearly five tons of food for the Khmer Rouge was unloaded by hand, taken to the Cambodian border, and then carried into Cambodia by men and elephants.

To try to solve this problem, the Red Cross and UNICEF have earmarked $9 million to buy trucks and boats this year and another $6 million for next year.

Daily flights are planned once the relief effort gets into full swing. The Red Cross and UNICEF are not sure when this will be.

However, there has been some question about whether the Cambodian government will let them deliver supplies to all Cambodians. According to UNICEF, the government has given a green light for aid to everyone in Cambodia, without restrictions. But this runs counter to a statement by the Cambodian Foreign Ministry Sept. 27 that it would not allow any international relief plan if it includes help to areas controlled by Khmer Rouge guerrillas. The Red Cross-UNICEF plan covers all those areas. UNICEF says it has not been told it cannot enter those areas.