Ukrainian officials said today that 84,000 people have now been evacuated from the region around the still smoldering nuclear reactor at Chernobyl, and a top Soviet scientist said crews were working underneath the reactor, the first indication by the Soviets that the molten fuel might break through the concrete floor into the ground.

Scientists in West Germany have expressed fears, based on the kinds of inquiries they have received from Soviet diplomats, that molten fuel might melt the concrete floor of the reactor, threatening the water table beneath the reactor and ultimately the water supply for Kiev, a city of 2.4 million about 80 miles away. Some U.S. scientists, however, were skeptical that the fuel had melted the floor.

The all-out efforts under way to keep the fire and its effects from spreading include dumping sacks of sand, clay, lead and the metalloid element boron on the reactor from above "to make people even safer against its radioactive poison," the Soviet newspaper Pravda reported.

A government statement by the official news agency Tass said work was continuing on shoring up the banks of the Pripyat River near the plant to prevent contamination of the water that flows into a reservoir that supplies Kiev.

Workers are also constructing earthworks near the plant to control the spread of radioactive debris, a senior western envoy said here today.

As the disaster at Chernobyl entered its 12th day and Soviet officials reported the increased evacuations from the vicinity -- up from 49,000 reported evacuated from four nearby towns last weekend -- the Yugoslav news agency Tanjug reported from Moscow that a third victim of the accident died today.

Thousands more residents poured out of Kiev into Moscow, after Tass indicated yesterday that radiation levels in the Ukrainian capital city had increased.

The ruling Soviet Politburo, in its first statement on the disaster, promised reparation payments to the evacuated individuals and enterprises, Tass reported tonight.

A Soviet worker brought from the disaster area died today in a hospital in Moscow, where about 200 of the injured have been evacuated, Tanjug reported. Soviet officials have said that two persons died in the accident, and 197 were injured, 18 seriously.

The government newspaper Izvestia today praised the workers "who are extinguishing the still smoldering embers of this fire."

Hans Blix, director of the Vienna-based International Atomic Energy Agency, who toured the area by air, said on Soviet television that "a little smoke is still coming up" from the damaged reactor. Blix arrived here Monday.

Reports of a continuing fire and additional evacuations contradicted earlier Soviet reports that the Chernobyl situation was under control, and raised the possibility of a lingering, uncontrollable nuclear meltdown.

But Ukrainian Prime Minister Alexander Lyashko told foreign journalists at a news conference in Kiev that "burning has practically stopped." He added that "the radiation is stable with the tendency downwards."

About 15 foreign journalists, selected by the Foreign Ministry, went to Kiev for a one-day visit.

Soviet rescue workers at the scene of the reactor fire are "working not only close to it, but also under it," Yevgeni Velikov, vice president of the Soviet Academy of Scientists said in an interview in the Communist Party newspaper Pravda. He did not elaborate as to why the crews were working under the reactor, adding only that "our specialists are mounting an offensive against the reactor. Our task is to fully neutralize the reactor, to deaden it."

Appearing briefly on the evening news program, Velikov added that "there is a lot of action to contain the reactor."

His statements gave the first official indication that the Soviet Union faces a possible long-term meltdown, with the reactor core sinking. West German nuclear specialists said yesterday that Soviet officials had inquired about methods of preventing a core meltdown.

"We are trying to solve problems no one has ever dealt with before," said Velikov, one of the Soviet Union's top physicists and an adviser to Communist Party leader Mikhail Gorbachev.

In the first acknowledgment that a second reactor had been endangered following the April 26 explosion, the official newspaper Sovietskaya Rossiya reported today that the roof of a building housing reactor No. 3 "caught fire, but the flames were successfully doused."

There are four reactors at the Chernobyl site. Western specialists here fear that a continuing meltdown could spread to reactor 3, bringing a second wave of increased radiation.

The town of Chernobyl was not evacuated until at least seven days after the accident, a senior western diplomat said here today. The town is located about 12 miles from the power plant and has about 30,000 residents.

"Our information is that at least in that town the evacuation was still going on two days ago," the diplomat said.

The evacuation of Chernobyl, the largest town near the reactor, did not begin until after the visit of Prime Minister Nikolai Ryzhkov to the Chernobyl area last Friday, the diplomat said.

Official warnings Monday appeared to touch off public anxiety in Kiev and an exodus from the city. Packed trainloads of people, particularly parents with young children, arrived in Moscow from Kiev today. Additional trains and cars were added to accommodate the flood of residents out of the area.

Some Kiev hospitals are treating local residents who took medicine "which was ostensibly supposed to protect them from radiation," Izvestia said. And stringent radiation checks are being made on travelers out of the city, a Soviet official told Izvestia.

Hector Cowan, a Canadian diplomat who returned from Kiev, the Ukrainian capital, said that train stations and ticket offices were jammed but that otherwise a mood of relative calm prevailed.

But Izvestia said that special measures had been introduced to protect the population, including stringent standards on produce, the prohibition of outdoor swimming and playing, and constant street sweeping.

Alarm rose Monday evening, following a television appearance by the Ukrainian health minister, who cautioned locals to stay indoors.

Officials here in Moscow were informed of the initial explosion on April 26, Lyashko told journalists in Kiev. But they were not alerted until April 28 about the full gravity of the situation, he said.

The Ukrainian leader also denied that human error had been involved and that the population outside the 18-mile zone around the reactor area is endangered.

"The accident developed in an unusual way," he said, "not as scientific knowledge would have predicted. The measurements at first showed there was nothing to fear."