The nuclear accident that is being called the worst in history began this weekend in a small town in the part of the Soviet Union that residents call the Polessa -- "the land along the edge of the trees."

But the fire, believed to be still burning at the Chernobyl nuclear power plant is releasing radioactivity that the winds and the nearby Dnieper River may carry across a 900-mile swath of the country's richest agricultural region that is part of the breadbasket of the Soviet Union.

Chernobyl is on the bank of the Pripyat River at the mouth of the larger Dnieper, 60 miles north of the Ukrainian capital of Kiev. About 68 million people inhabit this westernmost slice of the Soviet Union which stretches from Estonia in the north to the Ukraine in the south.

Winds have already carried a contaminated cloud northward, across a region where dairy animals are raised and grains farmed. The Dnieper River, Kiev's main water supply, may carry the radioactivity southward into the country's heavily populated agricultural heartland.

The Soviet Union has offered little information about the accident. The level of contamination is not yet known. Western experts, however, said that radiation from the accident will have long-term effects on the local population.

The Soviet government announced yesterday that two persons had died during what western experts suggested had been a partial or a complete meltdown of the reactor's core. Four towns near the plant were evacuated. Other, unconfirmed reports suggest a higher number of casualties. One report by United Press International put the total at about 2,000.

Although the region is primarily known for its agriculture, it is dotted by important cities as well: Kiev, Minsk, Riga, Odessa, Tallinn and Vilnius. Moscow is 465 miles to the northeast and Leningrad is 652 miles north. But most of the region's residents live scattered in thousands of small villages, where raising livestock and farming are still the way to make a living. The area's topography resembles a heavily populated Kansas or Nebraska, said Soviet specialist Robert G. Jensen, chairman of the Geography Department of Syracuse University.

"We're talking about one of the more heavily populated regions in the U.S.S.R.," he said. "It seems quite likely the results will become apparent in the food chain -- the cows eating the grass and so on."

The site of the accident, at the Chernobyl nuclear power plant, is on the southern border of the densely wooded Pripyat marsh area. The town is literally "along the edge of the trees," as the tree-covered marsh forms a 150-mile-wide natural border between the wooded north, where the land feeds dairy animals and yields grains, and the south, the "black earth zone," the country's most productive agricultural region.

In the north, population density is 25 to 50 people per square kilometer, said Jensen. In the south, it is higher, 100 to 200 per square kilometer.

"It's not rural in our sense," said Jensen. "These are not detached, single family farms, it's quite dense."

Chernobyl, the plant, and Pripyat, the company town built nearby in 1970 for workers at the plant, straddle the Ukraine's largest source of drinking and irrigation water, the Dnieper River. The river was dammed outside Kiev, the country's third largest city.

Swedish officials said they fear radioactivity from the accident may have contaminated the river, but there is no conclusive information to that effect.

Pripyat is a town "born of the atom," said a February edition of the Soviet magazine, Soviet Life. The average age in Pripyat is 26, it said, and the residents are "not disturbed by the fact that they can see the nuclear power units from the windows of their apartments."

"The units resemble a ship with white superstructures on deck. Radiating from the ship are the openwork pylons of power transmission lines," it continued.

The streets of Pripyat are lined with flowers, the apartments are nestled among pine groves, and in the morning, on the streets, "only young women pushing baby carriages stroll along unhurriedly," the article said.

In the same issue of the magazine, Vitali Sklyarov, minister of power and electrification for the Ukraine, said of plant safety: "The odds of a meltdown are one in 10,000 years."

The article said the reactor is housed in a concrete silo. "Even if the incredible should happen, the automatic control and safety systems would shut down the reactor in a matter of seconds. The plant has emergency core cooling systems and many other technological safety designs and systems," it said.

The plant is also a sort of eco-system. Its 20-square kilometer cooling pond is a warm water fishery that supplies fresh fish to Pripyat stores. The pond is constantly monitored, as is the surrounding flora and fauna, said Soviet Life.

Safety measures appear to be what comforted Boris Chernov, 29, a steam turbine operator. "I wasn't afraid to take a job at a nuclear power plant," he is quoted in the article as saying.

Pripyat is just 60 miles from Kiev, home of about 2.3 million people. The capital of the Ukraine, it is a city of both business and pleasure. The region's government bureaucracy has its offices in Kiev, and there are large machinery, clothing, paint, food-processing and chemical factories there. But half of Kiev is covered by 66 parks and 170 public gardens. Chestnut trees line the boulevards and there are 1,300 libraries, 77 movies and 116 cultural salons, according to a 1980 book on the city published by the Soviet government.

Kiev is one of Europe's oldest cities. It is believed to have been founded in the 6th or 7th century A.D. Older than Moscow, it is known as the "mother of Russian Cities."