The bountiful markets and generally healthy-looking people of Cambodia testify that the famine that plagued this country in 1979 is now little more than a painful memory.

But new developments suggest that Cambodia's food problems may be far from over, and some Western aid officials forecast the return of dire economic conditions next year.

Some of the current difficulties stem from the failure of two United Nations agencies to deliver vital rice seed and fuel on time, a sharp drop in food assistance promised this year by the Soviet Union and increasingly serious pilferage of foreign aid shipments.

A key future problem for Cambodia is the scheduled end of the U.N. emergency relief program at the end of the year. This means the loss of U.N. aid currently running at $335 million and totaling $636.4 million from the start of the emergency program in October 1979 to the end of 1981.

"Next year economically we'll see a worsening of the situation," a U.N. agency director here said. "Cambodia will be entirely dependent on an impoverished Vietnam and a Soviet Union which is not noted for its generosity."

Another aid official said Cambodia is unlikely to reach self-sufficiency in rice production this year as previously hoped. Moreover, officials said, there is little prospect that the amounts of food, seed, fuel and other commodities delivered by the United Nations can be made up from other sources next year.

A major stumbling block is that the new Cambodian currency, the riel, has no value outside the country. Even in Cambodia, it is worth only about a fourth as much at the black-market exchange rate as at the official rate.

"I don't see how they're going to buy the necessary fuel" for the distribution of rice seed and other key supplies, a Western aid official said.

He said the Phnom Penh government desperately needs recognition to qualify for assistance from such bodies as the International Monetary Fund and the World Bank. Recognition has been denied because the government was installed by invading Vietnamese troops that drove out the officially recognized Khmer Rouge government under Pol Pot in 1979.

Fuel is already in chronically short supply, causing widespread power cuts in the Phnom Penh area for several hours a day.

This, in addition to the lack of foreign exchange to buy raw materials and spare parts and the absence of diplomatic relations with Western countries, has seriously restricted the government's efforts to resume some industrial production.

At each of two factories visited July 15 on the outskirts of Phnom Penh, production had been halted by one or more of these difficulties. A textile and sandal factory employing 760 workers had been idle since June 22 because of power cuts and lack of chemicals and dyes.

"So all the workers are on leave," said plant director Bin Chhieng, "but we pay them anyway." He said lost production cost the plant potential earnings of $18,750 a day.

At a distillery built in 1928, production had been stopped for two months because an ancient French-made generator had given out and spare parts could not be obtained, the manager said. He said employes were trying to fashion a vital part in a metalworking shop by hand. Among the plant's products are pineapple wine and a rice-derived beverage called "Bayon whisky." Both plants operated, although not at full capacity, under the Pol Pot regime, their managers said.

Assistance for such industrial units has been excluded by the U.N. program, whose "limited objective" was only to help Cambodia overcome its famine and not to provide rehabilitation or development aid, a U.N. official said. Now the emergency is considered over, and the U.N. agencies here are going ahead with plans to pull out in December.

Phnom Penh government officials acknowledge the prospect of tougher economic times next year but insist that the problems can be overcome.

"If there is a reduction of international aid, especially U.N. aid, we think we will have economic difficulties," said a government spokesman. "But our land is rich, and we are used to adapting to a very hard life. So if there is a crisis, this crisis won't last long and it can't destroy our perseverance and vigilance. We can get by."

Vice Foreign Minister Hor Nam Hong said Cambodia would count on "the help of friendly socialist countries." He said that "up to now the socialist countries have always answered our appeals" and that he did not think Soviet aid would diminish.

In fact, the Soviet Union continues to be one of the Phnom Penh government's major aid donors, but Moscow's food assistance pledge for 1981 has just been scaled down by nearly 40 percent from the 90,000 tons of wheat, flour and rice promised at the beginning of the year, according to officials here.

This could aggravate food shortages that have already been reported in some provincial districts.

According to reports from the Thai border, thousands of Cambodians from the western provinces of Battambang, Siem Reap and Pursat have been coming to the frontier lately in search of food, saying they have consumed their rice stocks.

"If people come at this point and say they've eaten up all their stocks," one aid official said, "it's really a problem."

This development has helped to swell the refugee population along the Thai-Cambodian border to an estimated 125,000 people, up from about 90,000 a few months ago. Another factor has been a "relocation" program by the Thai government in which Cambodian refugees have been sent from camps inside Thailand to camps on or just across the border in the hope that they will then return home. As many as 20,000 Cambodians reportedly have been relocated in this way since the beginning of the year.

In addition, as many as 90,000 Cambodians remain in camps inside Thailand. The Thai government is seeking to repatriate these refugees with the tacit agreement of the Vietnamese, who launched an incursion into Thailand last year when the Thais forcibly repatriated 40,000 Cambodians. In the absence of an agreement, the two-way traffic to the border is making the area increasingly volatile.

Another potentially serious setback for Cambodia is the failure of the United Nations' Rome-based Food and Agriculture Organization to deliver at least 25,000 tons of rice seed in time.

The seed was meant to be in the hands of Cambodian farmers in early May, before the onset of the monsoon, but was not delivered until early July, officials here and in Bangkok said.

Compounding the problems have been the early arrival of the monsoon and the inability of the U.N. Children's Fund (UNICEF) to make timely delivery of several thousand tons of fuel, officials said.

In addition, Phnom Penh authorities were said to have neglected the food problem because of preoccupation with the recent international conference on Cambodia held in New York.

Much now depends on the vagaries of the current monsoon, Cambodian and Western officials agreed.

Yet another complication is mounting theft of aid shipments by black marketeers in collusion with corrupt Cambodian port workers and possibly other officials, aid sources said.

At the port of Phnom Penh, children can be seen waiting outside the walls of the unloading area for dockers to throw them bags of rice seed, which they then deliver to people across the street.

At the southwestern port of Kompong Som, 130 tons of fertilizer were lost recently to theft by Cambodian dockers. They were not stealing the fertilizer, it turned out, but the sacks it came in.

Some pilferage losses have been running at 10 percent of a total cargo, officials said, and one volunteer agency recently reported a loss of 30 percent.

Western aid officials believe these losses may continue to mount as the end of the U.N. program approaches.