U.S. officials now appear certain that the Chernobyl nuclear power plant experienced an extensive core meltdown. They also said they believe the nuclear accident was confined to a single reactor and that the fire at the site in the Ukraine may have been extinguished.

But a Soviet Embassy official, in an extraordinary appearance before a congressional committee here, made clear that the Soviets' problems are not over. The problem at Chernobyl, he testified, "has not been liquidated yet."

Some Reagan administration officials appeared to back away from earlier statements that hundreds of people may have been killed in the nuclear power plant accident. Immediate casualties were probably confined to those working at the plant, and many in the normal work force of 250 were probably evacuated from the site before the catastrophic explosion and fire, some officials now believe.

But the officials stressed that they still do not know the level of radiation around the plant and so cannot predict how many people will suffer long-term health damage. One official said that satellite photographs show tourist boats plying the river near the plant and soccer games within a few miles of it.

The Reagan administration also stressed its frustration with Soviet unwillingness to provide more information about the accident. Officials said they remain largely in the dark about its severity, levels of radioactivity in the area and the number of casualties.

But scientists at the Lawrence Livermore National Laboratory in California, in the most reliable calculations to date, confirmed yesterday that there had been an extensive meltdown. "The core is gone," one Livermore official said. The calculations were based on an analysis of emissions at the plant.

The nuclear accident, described as the worst ever, apparently began last Friday, but the Soviet Union did not announce it until Sweden detected abnormal levels of airborne radioactivity on Monday.

An administration task force headed by Lee M. Thomas, administrator of the Environmental Protection Agency, said that increased monitoring in the United States and Canada shows no abnormal levels of airborne radioactivity. The air mass containing radioactivity released during the initial accident late last week "is now widely dispersed throughout northern Europe and polar regions" and has reached the western coast of Norway, the task force reported.

U.S. officials continued to say that they do not believe the accident at the Chernobyl plant, 60 miles north of Kiev, will send dangerous levels of radioactivity over this country.

"Based on the latest information, there is no reason to believe that levels reaching this country pose any threat to the health and safety of citizens of the United States," Sheldon Myers, acting director of EPA's office of radiation programs, said. "It is very unlikely that significant amounts will reach the United States."

The testimony of Vitaliy Churkin, a second secretary of the Soviet Embassy here, was given at a hearing of the House energy, conservation and power subcommittee. During frequently testy exchanges with members of the subcommittee, Churkin would not say whether the fire has been extinguished, but appeared to indicate that it continues to burn.

"The problem is getting better. It is not out of hand. It is improving," Churkin said. "But unfortunately, it is not over yet."

Churkin, saying the Soviets have been "very forthcoming," bristled at suggestions that his government misled the world about the severity of the accident and misled its own population about the danger of radiation. He said two people were killed in the accident and 197 injured, 18 of them seriously.

"It was a horrible tragedy," he said. "All those who suffered and are suffering will be taken care of."

One knowledgeable U.S. official said the Soviet casualty figures are now thought possibly to be correct, although the administration continues to harbor considerable skepticism. Shortly after the world learned of the accident, U.S. Arms Control and Disarmament Agency Director Kenneth L. Adelman said the Soviet casualty figures were "preposterous" and State Department spokesman Charles E. Redman said that hundreds of deaths were possible.

One Defense Department official said that U.S. officials have no good evidence to contradict the Soviet statements but neither can they confirm them.

"Our intelligence is as good as it can be without any cooperation from the Soviets," the Pentagon official said.

Officials said they believe that radiation levels in the Chernobyl area, given the severity of the fire and blast, must be high enough to warrant a greater evacuation than the Soviets appear to have ordered.

"Either there's nothing seriously wrong, which we don't believe, or there's something very seriously wrong, but to carry out their lie they're willing to go to the extent of hazarding the lives of people around the plant," the Pentagon official added.

But Churkin told the House subcommittee that all those affected "were well aware of what has happened."

"They won't even have to pay medical bills," he added.

Intelligence experts here also remain divided about whether the fire in the reactor is still burning. The question is important because the amounts of radioactivity released into the atmosphere may diminish once the fire is out.

Officials said that Soviet helicopters were actively fighting the fire Wednesday, dumping sand, chemicals or some other substance on the reactor. Experts said that as long as the levels of radioactivity were low enough to permit helicopters to hover nearby, the Soviets could have smothered the fire in the nuclear plant's graphite rods.

But Les Williams of Boots and Coots, a private fire-fighting firm in Houston, pointed out that it would be very difficult to determine from the air whether the graphite fire is extinguished.

Graphite, if not contaminated by other materials, burns without smoke, he said. Early in the accident, building materials and other debris including nuclear fuel would have yielded a dark smoke, visible in satellite photography. But later, as the fire became more purely graphite in burning, the smoke would dissipate.

In addition, satellite heat sensors would detect a very hot spot as the graphite slowly cools, even once the fire had stopped.

A second hot spot detected from space Tuesday had prompted speculation that an adjoining reactor had caught fire. But U.S. officials said the second heat source is outside the plant and apparently unrelated to the nuclear accident, speculating that it is a construction facility of some kind.

Thomas of the EPA said U.S. experts "say that it is plausible the fire could be out" but cannot confirm that. Harold R. Denton, a Nuclear Regulatory Commission official, said that even if the fire were extinguished, significant amounts of radioactivity will remain in the area.

"They have a huge problem with regard to cleanup," said Denton.

Technical expertise in decontamination was part of the assistance that President Reagan offered and the Soviets declined, according to Energy Secretary John S. Herrington.