Mikhail Gorbachev is asking the world to help his country cope with the multibillion-dollar legacy of contamination that the Chernobyl nuclear power plant unleashed five years ago today.

A radioactive cloud rose over the Soviet republics of Byelorussia and the Ukraine and moved quickly across Europe after the 1986 blast and ensuing fire in Reactor No. 4.The Soviet government says the disaster killed 32 people: two immediately, 28 within three months and two more in the past five years. But some Soviet scientists and radical politicians in the Ukraine say the actual death toll was at least 250 and could be as high as 10,000.

The Soviet government blamed the steam explosion on human error during a "routine shutdown" of the reactor, about 80 miles north of Kiev. But many Soviet and Western experts also fault the design of the 1,000-megawatt RBMK reactor, which is not permitted in most Western countries.

On the eve of the fifth anniversary, Gorbachev appealed for international aid to deal with the disaster's long-term effects.

"The Chernobyl tragedy has not become a thing of the past," the Soviet president said Thursday. "Mankind is just beginning to realize fully the global nature of social, medical and psychological problems created by this catastrophe."

Gorbachev's "Appeal to the Governments of All Countries and World Society" was read by a newscaster on Soviet television and carried by the state news agency Tass.

Soviet health and energy officials announced this month that the government has spent more than $16 billion on the Chernobyl cleanup. They estimated that the full cost of the disaster, including damage to crops and the value of abandoned buildings in the contamination zone, could be as high as $350 billion.

In his message, Gorbachev thanked the United Nations, the World Health Organization and the many countries and individuals that have supported Chernobyl relief programs. But he said more help was needed.

Gorbachev noted that the World Health Organization is planning an international program to study the medical aspects of the accident and construct a radiation research center in the Soviet Union.

Dr. Robert Gale, an American surgeon who performed bone marrow transplants on Chernobyl victims, has predicted that 2,000 to 20,000 people will die of cancer during the next 50 years resulting from radiation released at Chernobyl.

About one-third of the deaths will be in the Soviet Union, half in northern Europe and the rest spread around the world, he said.

For nearly three days after the accident, the Soviet government said nothing about it, until Nordic countries detected the radiation and demanded an explanation. Gorbachev then invited foreign experts to the scene, a major step in his policy of glasnost, or openness.

More than 180,000 people were evacuated from homes in the Chernobyl region, a fertile agricultural area. About 1,000 have moved back, ignoring appeals from authorities.

In addition, at least 4 million people live in the 52,000 square miles that the government says were hit by radioactive fallout - an area the size of Illinois.

The Ukraine, often called the Soviet Union's breadbasket, continues to produce grain and other crops on soil that was exposed to radiation.