The Soviet government has ordered 20,000 evacuations from additional areas as a result of the nuclear accident at Chernobyl in the Ukraine and has extended the radiation danger zone into parts of neighboring Byelorussia, officials indicated in a news conference here today.

Yuri Israel, chairman of the state committee for hydrology and meteorology, told reporters that high temperatures lifted the radioactive emissions high into the air, where winds swept them far afield after the April 26 explosion.

"That was responsible for certain levels of radiation outside" of the original 18-mile closed danger zone, Israel said.

"We have evacuated the population of additional villages and towns," Yuri Batalin, a deputy prime minister, added later, "and we determined new boundaries of contaminated areas."

The 90-minute press conference to discuss the Chernobyl accident, held by six senior Soviet officials, came a day after the Communist Party newspaper Pravda reported that additional "dirty spots" of radiation had been discovered in Byelorussia, the republic bordering the Ukraine on the north.

Pravda said 60,000 children had been sent out of the danger zone in Byelorussia, and today A.M. Petrosyants, chairman of the state committee for atomic energy, said that 20,000 people had been evacuated from "additional areas" contaminated in the disaster.

Previously, officials had said 92,000 people were evacuated from the area. It was not clear whether the figures of 60,000 and 20,000 mentioned today were in addition to the 92,000 or included. Officials at the press conference, in reporting the discovery of new pockets of radiation and new evacuations, gave no clarification or overall total of evacuations to date.

The expansion of zones of radiation and increased evacuations appeared to contradict recent assurances by Soviet officials that the Chernobyl disaster, now entering its 10th week, is under control. Some of those originally evacuated have returned to the Chernobyl vicinity, Israel told journalists, and "today, tomorrow, others are returning."

But the ruling Politburo, in a meeting today, introduced new measures to "intensify" the Chernobyl rescue operation, the official news agency Tass reported, indicating that Kremlin involvement with the worst disaster in the history of nuclear power is not waning.

Other officials reported an increase in the Chernobyl death toll and renewed efforts to protect the local water supply. They also acknowledged today that radioactive gas is still being emitted from the damaged reactor and that it may not be possible to fully decontaminate some affected areas near the plant.

"Most territories will be brought back to normal economic life," Israel said, "but it is not excluded that an insignificant amount of territory will be used for continued monitoring."

Israel also said that the amount of radioactive debris emitted into the atmosphere following the accident ranged from 1 to 3 percent of the radioactive product of the reactor, but he left unclear the exact amount of debris in question.

The death toll has risen to 26, one more than reported earlier this week, according to Oleg Shchepin, deputy health minister and one of press conference participants.

Shchepin said all of those still hospitalized were admitted in the first two days after the accident. Another Soviet official said Tuesday that 18,000 persons evacuated from the area were admitted to hospitals for radiation checks immediately following the incident and released.

Israel said that the "new round" of rescue work at Chernobyl, likely to continue until the end of the year, involves burying the reactor and isolating it from underground waters. The Politburo also endorsed an "intensive effort" to "protect bodies of water from contamination," Tass reported today.

The high-level attention directed toward the water supply in the region raised new questions about the extent to which the nearby Pripyat River and other key waterways were threatened by the disaster. Israel said that "a lot of measures" are being used to finally "cut off these ground waters completely from the reactor area."

The officials also indicated that radiation levels in several Soviet cities, including Kiev, the country's third-largest, 80 miles from the Chernobyl reactor, are still above the normal background level.

Deputy Prime Minister Batalin said that special housing is being built for some of those evacuated, "and when it is possible to resettle them in their original homes, we'll resettle them again."

Another Soviet official said that they were resettled in an area 25 to 35 miles from the original settlement.

Batalin said that "significant" economic damage had resulted from the accident, in terms of lost electrical energy and the cost of resettling those evacuated. The latter, he said, would involve "big sums."

Shchepin today rejected western embassy reports of contaminated milk and meat products in Moscow, saying that "additional efforts are being made" to prevent the shipment of contaminated food from the region.

He said finding contaminated food in stores in the Soviet capital is "absolutely impossible."