A leading Kremlin official said tonight that human error was the cause of the nuclear accident in Chernobyl and that radioactivity from the plant was still preventing crews from entering the facility.

Moscow party chief Boris Yeltsin, offering the most extensive details yet on the accident and the plant's present condition, said in a West German television interview that "there is still radioactivity around the plant. There is a dangerous zone which people cannot enter."

In the interview, details of which were made available here, Yeltsin was quoted by a television announcer as having said that water reservoirs in the vicinity of the plant had been contaminated and that authorities were seeking to prevent the spread of radioactivity by dropping lead, sand and boron into the area from helicopters.

He said four settlements close to the plant had been evacuated and that the level of radioactivity, although fallen considerably since the disaster occurred, was still around 200 roentgen per hour, more than 300 times the lethal dose.

In London, a government official said the Soviets had asked the British government for assistance. The British are believed to have specialized equipment to handle such disasters.

In Kiev, 60 miles south of Chernobyl, Canadian diplomat Hector Cowan, the first western diplomat allowed to travel to the vicinity of the blast, said in a phone interview that the "situation is apparently normal." He said buses were running and there were no signs of panic or health problems.

Yeltsin, a candidate member of the Soviet Communist Party's ruling Poliburo, made his comments in an interview with the ARD television station in Hamburg, where he is attending the West German Communist Party conference. In a speech to the conference, he also criticized western news organizations for reporting "outright lies" and "horror stories."

Yeltsin's revelations about the plant came as the countries nearest to the Soviet Union were anxiously following the directional shifts of the radioactivity released by the plant last weekend and still not dissipated. The winds that carried the contaminated air north into Scandinavia earlier this week have changed, carrying heavier concentrations of the radioactive fallout into Central Europe in recent days. Details on Page A14 .

The unpredictable air-borne danger has also put embassies in the country on the alert. Several have introduced new measures to protect their citizens here, including warning against consuming local produce, and in some cases offering to evacuate dependents to safer locations.

Senior diplomats from Japan, West Germany, Denmark and several other countries counseled their nationals against drinking Soviet milk, which they fear may come from radiation-contaminated cows. They also urged careful washing of vegetables.

Sweden plans to fly alarmed dependents back to Stockholm, and Canadian and Australian officials are weighing similar moves, according to diplomatic sources here.

Radiation tests from Austrian, British, French and Finnish citizens evacuated from the Ukrainian city of Kiev and other areas in the vicinity of the nuclear incident showed "slightly higher than normal," effects of radiation, "but nothing hazardous," a well-informed western science specialist said here today. Besides a handful of "quirky radiation readings," there was nothing to cause alarm, the expert said.

Robert P. Gale, a prominent American bone marrow specialist, arrived here today from the United States to assist in the treatment of Soviet radiation victims. The Soviet news agency Tass reported Wednesday that 148 Chernobyl victims remain hospitalized. On Thursday Tass announced that 18 of them are in serious condition. No foreigners are affected, Tass said.

Although U.S. officials import shipments of milk and produce from Finland weekly, the American Embassy, in a statement issued today, said that local dairy and produce were consumable unless radiation levels in the Soviet capital increased. The radiation here has remained stable since the nuclear disaster last Saturday, according to local readings.

Other U.S. medical specialists are expected to come to Moscow this weekend to investigate the health dangers resulting from the disaster.

In a meeting here today, Androiac Petrosyants, head of the Soviet Atomic Energy Commission, assured Austrian officials that the Chernobyl situation was fully under control.

Petrosyants said there had been only one blast of radiation from the Chernobyl plant, and that the high readings that followed were the result of it, an Austrian Embassy spokesman said.

Petrosyants declined to answer lingering questions about the cause of the accident, saying that a full report will be released. Diplomats expect that the report, under preparation by Soviet nuclear experts, will be submitted to the International Atomic Energy Agency, based in Vienna.

Soviet leader Mikhail Gorbachev, in his first public statement released after the incident, said today that the Soviet Union is "in no hurry to resume" nuclear weapons tests. In keeping with the tight lid on information, he did not mention the Chernobyl disaster.

In a letter to the leaders of Argentina, India, Mexico, Sweden, Tanzania and Greece, Gorbachev said, "we are prepared in any moment to return to the question of a nuclear moratorium if the United States does not conduct nuclear tests."

The text of Gorbachev's letter was released by the Soviet news agency Tass amid indications that senior officials in the Soviet capital were only informed about the incident last Monday, two days after it occurred. There were no official reports about the incident today.

Instead, state-controlled television attacked western media coverage of the incident. A report on the evening news charged that the western press had mounted a campaign of "slanderous inventions" about the Chernobyl situation. The evening news broadcast featured several interviews with British, French, Canadian and African tourists and students complaining about how the western press had exaggerated the dangers of the nuclear incident.

The broadcast included no information about the situation in or around Chernobyl.

Western diplomats and journalists have been barred from the area, but Canadian diplomat Cowan was allowed to leave for Kiev on Thursday to assist about 40 Canadians depart the city. Cowan said the Canadian students there have been living in "an information vacuum," since the incident occurred.

Caught between "bland reassurances from Soviets," and "quite alarming reports on western radio," they are frustrated and apprehensive, he said in a telephone interview from Kiev. Most of them will be leaving, he added.

The United States, Finland, France and other western countries have reported that all their citizens who have wanted to leave the area have left.

Washington Post correspondent Karen DeYoung reported from London:

The Soviet Union informally requested British assistance in dealing with the nuclear disaster, an Energy Ministry official said today.

The approach came from Soviet diplomats at the International Atomic Energy Agency to their British counterparts in the organization, a source said. The Soviets, apparently fearing further negative publicity about the incident and anxious to dampen ongoing speculation, have requested that the British keep the nature of the requested assistance confidential.

Britain has had experience in putting out a fire in a graphite-moderated reactor, similar to that at Chernobyl, in a much smaller scale accident that occurred at its Sellafield, formerly Windscale, plant in western Cumbria in 1957. The British also are believed to have specialized equipment and protective clothing designed for such emergencies