The Soviet Union urgently sought western help today in coping with a nuclear power plant disaster that outside experts described as the worst in history, but authorities here continued to make few details of the accident available and portrayed the situation as under control.

Official accounts here and intelligence reports reaching the United States and other western countries painted vastly differing pictures of the scope of the accident, which the Soviets acknowledged had destroyed part of the building housing a nuclear reactor at the huge Chernobyl generating plant 60 miles north of Kiev.

The first official Soviet statement on casualties said tonight that two persons had been killed "during the accident," and a Soviet official said in the United States that "fewer than 100 are dead."

Kenneth L. Adelman, director of the U.S. Arms Control and Disarmament Agency told a Senate committee in Washington, however, that the Soviet figures were "preposterous" and that the damaged reactor was still burning "at a fantastic temperature" that posed a continuing threat of further explosions and contamination.

United Press International quoted a source in Kiev as saying that more than 2,000 people had died in the accident -- a figure that could not be confirmed -- and the Soviets said four towns in the area had been evacuated.

*In Bali, Indonesia, presidential spokesman Larry Speakes said President Reagan was informed of the accident Tuesday and had ordered the creation of an inter-agency task force, headed by Environmental Protection Agency Adminstrator Lee Thomas, to monitor the aftermath. Speakes said the latest U.S. information indicated a radioactive air mass was passing over the Soviet Union and was expected to disperse in the atmosphere and that it was too early to tell whether the radiocactive cloud would reach the United States.

Speaking Wednesday morning to reporters, Speakes said the EPA has increased sampling of air in the United States on a daily basis. He said information indicates a fire in the graphite core of the reactor had destroyed most of the core. He said the reactor contains 200 tons of uranium and 1,700 tons of graphite. The graphite, he said, could burn and spread more radiation "and fighting the fire will be extremely difficult."

He said Assistant Secretary of State Rozanne Ridgway met with Soviet charge d'affaires in Washington, Oleg Sokolov, to express U.S. regret and to offer humanitarian and technical assistance. Reagan had also sent a personal message to Soviet leader Gorbachev offering assistance. The Soviets apparently have not yet responded to the these offers.

Sen. Malcolm Wallop (R-Wyo.), after attending a briefing by intelligence officials, said there had apparently been an "enormous explosion," and that the radiation level at the site was "up to a million times" that at Three Mile Island in Pennsylvania, site of a partial meltdown in 1979 that was the most damaging U.S. nuclear power plant accident. It caused no casualties.

In an indication that there were still serious problems, Soviet officials today asked nuclear safety experts in Sweden and West Germany for advice in fighting a graphite fire, an extremely hazardous byproduct of a nuclear accident.

Officials in both countries -- as well as the United States -- offered to assist, but they also expressed dismay at the Soviet delay in alerting nearby countries of the disaster so as to allow them to prepare for the fallout.

Radiation levels in Scandinavian countries, where abnormally high readings on Monday alerted the outside world to the disaster, dropped today as winds shifted.

But Poland, which borders on the Soviet Union, said today that it had detected extraordinarily high levels at 200 special monitoring sites. Polish authorities restricted sales of milk from cows that may have eaten grass contaminated by radioactive fallout, and, in northeast Poland, the government said it would distribute iodine tablets to children to counter the effects of radiation.

Tonight's Soviet government statement said that "according to preliminary data, the accident took place in one of the areas of the fourth power-generating unit and resulted in the destruction of part of the structural elements of the building housing the reactor, its damage and a certain leak of radioactive substances. The three other power-generating units have been shut down, they are in order and in the operational reserve. Two persons were killed during the accident."

It said: "Priority measures have been taken to deal with the effects of the accident. The radiation situation at the electric power station and the adjacent territory has now been stabilized and the necessary medical aid is being given to those affected. The nearby populated localities have been evacuated.

"The state of the radiation situation at the Chernobyl nuclear power station and the adjacent territory is being monitored continuously."

The statement also said that "a government commission headed by Boris Shcherbina, a deputy chairman of the U.S.S.R. Council of Ministers, is working in the locality. It includes the heads of ministries and departments and leading scientists and specialists."

The government statement, read on the evening news tonight without pictures or commentary, and coming after unrelated items on local agriculture, was the first official mention of the accident since 24 hours earlier, when a brief televised statement first announced it.

Western experts believe that a chemical explosion occurred at the plant resulting in a massive release of radiation that could continue for days. "People in the immediate area must have been exposed to God-awful levels of radiation," said one western diplomat, "and I am sure it "will show up in cancer rates in 10 years."

The accident was not mentioned in Soviet newspapers this morning, and most Soviet citizens, asked about it, said they were unaware of it. Yesterday's statement did appear in tonight's Izvestia, the government newspaper.

Tonight's statement was also spare, and seemed designed to play down the incident for the public. For instance, the names of the four towns that were evacuated were not given, nor were the names of the two dead, and there was no explanation of how or when they died or when the accident happened. There was no estimate on the number injured or hospitalized, and no information on possible long-term health effects.

Although little information was provided, just the admission that deaths had occurred alerted Soviet viewers to the severity of the event. "If they say that much, it means it is probably worse," said one office worker.

Accounts from Kiev, a city of 2.3 million, were conflicting and shed little light on the disaster 60 miles to the north.

UPI said a Kiev resident with contacts among hospital and rescue officials told the service by phone that more than 2,000 people died, many more were hospitalized with radiation sickness in Kiev and between 10,000 and 15,000 people were evacuated from Pripyat, site of the power station. But a spokeswoman at the main Kiev hospital denied any accident victims were there.

A high Soviet official who arrived in Washington yesterday disputed the reports of heavy casualties.

Michail Timofeev, deputy minister for civil aviation, held a press conference held at Dulles International Airport to mark the resumption of civil air traffic between the United States and the Soviet Union and said that "fewer than 100 are dead."

He acknowledged that it was "an accident but not a catastrophe" and would not discuss it further.

Travelers and residents reached by telephone reported life apparently continuing normally in Kiev.

U.S. Air Force Col. Robert Berls, an attache at the American Embassy here, was in Kiev yesterday, The Associated Press reported, and said on his return here that "no one was aware of anything. No one seemed upset or concerned at all. Activity . . . was completely normal."

The Manchester Guardian said Berls was checked with a Geiger counter at the U.S. Embassy, which reported a "modest, and far from lethal dose of radiation."

Sue Parminter, one of 70 British exchange students who are currently in Kiev, told the Guardian by telephone that she first learned of the disaster from a British Broadcasting Corp. newscast.

"I rushed straight to the university where I work," she said, "and asked Russian colleagues what was happening and what they knew. But they did not know what I was talking about. There has been no panic, no sign of crowds at the bus or train stations. The place just does not seem to know what has been hitting them for the past two days."

A telephone operator at Pripyat, about 10 miles north of the Chernobyl plant, told one western reporter that there had been an evacuation, but she was then cut off, Reuter reported.

Foreign embassies said they were getting calls today from tourists and others in Kiev, seeking information. The U.S. Embassy reported receiving about 10 such calls, and a spokesman said "quite a few" tour groups were in the region.

The Soviet Foreign Ministry told foreign embassies here that no foreigners had been injured.

The Soviet Union is about to start a four-day holiday that begins with May Day, May 1, and ends May 4, the Russian Orthodox Easter. Diplomats here anticipate that little new information will be provided over that period.

Attempts today by western diplomats and journalists here to get more information on the accident have been met with vague promises for followup information "as it becomes available."

Official requests for interviews with nuclear energy officials were refused, and a call to the Chernobyl city government today was taken by someone who said everything in the area was "normal," that there had been no casualties and no one had been evacuated.

Western diplomats and journalists are being denied permission to travel to Kiev, although the Soviet tourist agency Intourist is still booking trips.

A U.S. consulate is scheduled to open in Kiev this summer as part of an agreement reached between President Reagan and Soviet leader Mikhail Gorbachev. No other western country has a consulate in the Ukrainian capital.

The Chernobyl power plant, commissioned in 1977, is located on a reservoir north of Kiev on the Pripyat River, which flows into the Dnieper. The plant's location has raised concern about the effect of radiation on the local water supply.

So far, embassies said, no tourist has reported receiving any warnings from Soviet authorities about not drinking the water in Kiev.

The British Embassy, however, warned British citizens in the area to take antiradiation precautions, such as staying indoors and showering and washing their hair every two hours to get rid of radioactive dust, the Guardian reported. Those who live in Kiev were told to drink only boiled water.

The Soviet Health Ministry told the U.S. Embassy that radiation levels in Moscow have been normal.

Several embassies here called on the Soviet Foreign Ministry with specific requests for information about the accident.

The Swedish Embassy submitted questions and, according to diplomatic colleagues, made a strong oral protest over the Soviets' failure to notify nearby countries promptly of the hazardous situation.