Officials of both Sweden and West Germany expressed anger and dismay today at the failure of the Soviet Union to inform them about the nuclear accident that has ravaged the Chernobyl power station.

The complaints coincided with requests from Moscow to the two western countries for advice on how to deal with what apparently is a continuing fire at the facility.

Based on the contents of the fallout and the 600 to 700 miles it traveled through the air to Scandinavia, officials at the Swedish Nuclear Power Inspectorate said they believed that at least a partial meltdown of the fuel core in one of Chernobyl's four reactors had occurred.

They said they believed a fire was still burning at the site, and that it would be "a long-term process" of perhaps weeks to "return the situation to normal" at the reactor itself.

Officials from the Soviet Embassy in Bonn approached the Atomic Forum, a German nuclear research center, and private firms involved in building nuclear plants to seek advice on how to fight a graphite fire that appears to be burning out of control, West German officials said.

They said the information provided by the Soviets indicated the reactor core had melted down and probably had released enormous amounts of radioactivity.

The Swedish government angrily requested Soviet clarification of the accident and rushed to reassure its own citizens that they were in no danger from the significantly increased levels of wind-blown radiation it has brought here.

A Foreign Ministry spokesman said the Swedish ambassador to Moscow, along with his Danish colleague, had this morning presented Soviet authorities with a list of "very specific and technical questions formulated by experts in the field."

"Our main concern is to know what happened," said ministry official Ulf Hakansson. "We don't know the situation now at the nuclear plant. We don't know if it will happen again."

Thus far, no satisfactory response has been received, Energy Minister Birgitta Dahl said today. "The question of Swedish-Soviet relations depends upon the Soviet answers."

Bonn's Interior Minister Friedrich Zimmermann today offered Moscow any expertise and equipment necessary to cope with the disaster. But he also sharply criticized the Soviets for failing to notify neighboring countries about what had occurred at the Chernobyl plant.

Other top West German officials, including Research Minister Heinz Riesenhuber, called on the Soviet Union to open more of its nuclear power plants to international inspection to rectify safety hazards that were shown in the latest incident to pose serious threats beyond Soviet borders.

Although a chance alteration of wind direction -- from the southeasterlies that brought the radioactive cloud to Sweden over the weekend, to prevailing northerlies today that sent it back across the Black Sea -- have lessened the danger for the moment, Swedish officials warned that the threat is not over.

Should the wind change again, said Gunnar Bengtsson, director of the Swedish Institute of Radiation Protection, "then we could definitely receive more radiation. We are watching the situation very closely."

Frigyes Reich, a senior engineer at the Swedish agency, said his office had "today received a visit from the technical attache of the Soviet Embassy. The Russians want advice on how to put out a fire.

"We told them three things," Reich said. First, to close down the other three Chernobyl reactors immediately. Secondly, "a supreme effort must be made to cool the damaged core, and they have to attempt to contain the spread of radioactivity."

At this stage, Bengtsson said, "They don't appear to be calling for personnel, but more for expert advice, tips, some proposals."

Officials at the International Atomic Energy Agency in Vienna said they were poised to send an emergency team to the stricken plant as soon as Moscow asked for help. But a spokesman said that as of Monday night the agency had received no aid request from the Soviet Union.

Sweden also referred the Soviets to Great Britain, where a similar accident of far lesser magnitude occurred in 1957 at the British reactor then called Windscale, in western Cumbria. The British reactor, now part of a massive nuclear reprocessing plant called Sellafield, has suffered a series of minor accidents since the 1957 crisis, and is the subject of heavy domestic criticism.

A British government spokesman said tonight that they had received no request from the Soviets, "no approach in London or Moscow." Environment Secretary Kenneth Baker said Britain was "ready to give assistance" if the request came.

In a statement today to the House of Commons, Prime Minister Margaret Thatcher rejected any lessons for Britain's nuclear program from the Soviet crisis. "We have very high standards of safety," she said.

Swedish authorities believe the Soviet accident happened Saturday morning, or perhaps even as early as Friday. Although unmanned Swedish monitors began recording the increased radiation on Sunday, no one checked them until Monday, when radiation levels in populated areas initially led to fears here of an accident at a Swedish reactor.

Measurements yesterday indicated a tenfold increase in radiation levels in many places, and even higher levels in certain areas. Since then, government switchboards have been jammed with calls from fearful Swedes asking whether it is safe to go outside or drink water and milk. A number apparently are following the course of one young Swede who today said he was sending his pregnant wife out of the country on a flight to Ireland out of fear of possible radiation-induced birth defects.

Officials said today that, as radioactive debris was brought to earth by rain and snowfall on Sweden's east coast, radiation levels had decreased in the air from yesterday's highs. But ground levels had, in some parts of the country, gone up to as much as 100 times normal. They insisted, however, that most of the isotopes contained in the fallout had brief half-lives of only a few days and that, barring a new dose of radiation, the levels should now fall drastically.

But despite reassurance to the population, officials here did not hesitate in calling the accident "the gravest in nuclear history."

Asked why officials here believe a meltdown, or partial meltdown has occurred, Bengt Petterson, a senior official at Sweden's Nuclear Power Inspectorate, said the assessment was based "essentially on the information on radio nucleides being deposited in this country," including high concentrations of cesium and iodine. "That is Exhibit A," he said. "I think Exhibit B is the long distance and the wind direction that happened to bring the stuff to our country."

"It could probably be termed as something with significant core damage," Petterson said. "Obviously in such a situation, you have to evacuate people . . . I would guess they would have to evacuate people within a hundred kilometer radius."

Swedish officials were highly critical both of longterm Soviet refusals to allow international atomic inspectors into their civilian power facilities, and the lack of safety features on Soviet reactors, particularly the lack of containment structures built around the reactor.

"In my opinion, and that of the Swedish government," Energy Minister Dahl said, "it is unacceptable to pursue a nuclear program with such low safety requirements.

"I hope the Soviet Union will learn a lesson from this extraordinarily brutal experience. It is up to the Soviet Union to protect its own population."

As for Swedish citizens, the Radiation Protection Institute today estimated that, as a result of the accident, they will receive two to three times their average annual dosage of radiation this year.

Natural, or "background," radiation already is higher in Sweden than in most other countries because of a vast amount of natural uranium in the ground.

Asked how many people here would be affected by the fallout, Jan Olof Snihs, a senior official of the government institute, said "more or less all. But in varying degrees, of course." Even if the level does even out to treble the normal yearly dosage of 60 millirems, however, he said, it still is only a fraction of the amount normally considered safe for nuclear industry workers.

In addition to taking air and soil samples, the government is studying iodine content in both mother's milk and bottled milk. "Iodine is going to the thyroid," Snihs said. "Since it's also used by small children, we would like to have this calculation about the doses. Not because we think it's dangerous to babies, but because that's one of the important exposure pathways."

According to institute director Bengtsson, "the measurements we have so far are only one one-hundredth of what we would consider to be a situation where we would have to take action.

"Of course, it is unpleasant, and we wish it wasn't here. But it's not a level where we need to take special action."

As the government awaited answers from the Soviet Union, Swedish newspapers today were harshly critical of Moscow's actions. The afternoon Expressen said that the crisis was one of "not national, but international concern . . . An early warning from the Soviets would have saved her neighbors much worry."

The Aftonbladet, closely allied with the ruling Social Democratic government, noted that "no country's atomic power policies can be isolated from that of other countries."

Most papers here published lengthy critical inquiries into Sweden's preparedness for such emergencies. Questioned at a news conference as to why warnings from Sweden's atmospheric measuring devices were not heeded on Sunday, when the high-level radiation began to be recorded at about noon, Snihs said that, "There was no alarm in Sweden because we do not actually have a proper alarm system functioning."

He said the system of unmanned recorders currently was being computerized "so that in the near future, alarms will be given automatically . . . conveyed directly to Stockholm.

"We're developing it at the moment, and after this event, you can be assured that the work is going to go a lot faster."

The crisis also has revived a long-smouldering controversy over Sweden's own system of 12 nuclear reactors that produce up to 40 percent of the country's power. A prominent Social Democratic official recently resigned to protest what he said was government waffling on fulfilling a 1981 referendum vote to close down all the reactors by the year 2010.

Norway and Denmark, two other recipients of the Soviet fallout, have no nuclear power program.Washington Post correspondent William Drozdiak contributed to this report from Bonn.