MOSCOW (AP) _ An American doctor treating victims of the Chernobyl nuclear disaster said today 80 patients are in serious condition, more than double the number reported three weeks ago.

Dr. Robert Gale, a U.S. bone marrow specialist, spoke at a news conference after returning from a three-day trip to the Ukraine where he visited Kiev and the town of Chernobyl, about 11 miles from the plant.

Gale said he was reluctant to predict future cancer rates as a result of radiation exposure. He said about 100,000 people would be monitored ''for the rest of their lives.''

He said doctors were ''seriously concerned'' about the prognosis for 80 radiation sickness patients, adding 78 of them had been at the Ukrainian power plant April 26 when a fire and explosion wrecked the No. 4 reactor and sent radiation into the air.

Of the remaining two patients, both residents of the nearby town of Pripyat, one was exposed to radiation through soil and the other was bicycling in a contaminated area, Gale said.

At a May 15 news conference, Gale said 35 people were in serious condition. Figures provided by Soviet official suggested the 2.4 million Kiev residents received doses of radiation equivalent to about half the amount of radiation they would normally be exposed to in one year, Gale said.

He said radiation levels in Kiev, 80 miles south of the plant, were 15 to 30 times above normal background level.

At a news conference Thursday, Oleg Shchepin, the first deputy health minister, said 26 people have died as a result of the accident. Officials said that of 299 people originally hospitalized for radiation exposure, 187 still were receiving treatment.

Dr. Leonid Ilyin, the director of Moscow's No. 6 hospital where many accident victims are being treated, said 230 teams of doctors and other medical personnel were working in the Ukraine and the neighboring republic of Byelorussia to monitor the effects of radiation on thousands of evacuees.

Ilyin also said workers decontaminating the Chernobyl plant are being monitored and pulled off the job as soon as they have been exposed to a dose of 25 roentgens.

''According to voluminous research by specialists from different countries, (this dose) cannot be dangerous to health,'' he told Moscow News, an English-language weekly.

Meanwhile, fires fueled by two months of dry weather were reported near the Ukrainian-Byelorussian border area. The youth daily Komsomolskaya Pravda said four soldiers have been decorated for their part in stopping a large peat fire near the village of Pogonnoye, whose exact location was not specified. It said forest fires also had burned, but gave no other details.

On Thursday, Deputy Premier Yuri Batalin told a news conference that about 20,000 people were evacuated from contaminated areas recently discovered in southern Byelorussia.

Pravda, the Communist Party newspaper, said Wednesday that the ''dirty spots'' were found in the Gomel area of southern Byelorussia beyond the 18- mile danger zone declared around the plant earlier, but gave no specific locations.

Official reports have said 92,000 people were evacuated from the danger zone within a week after the accident.

Pravda said about 260 families would be allowed to return to villages within the 18-mile zone because no health risks were found in their hamlets, and Batalin said people eventually would return to most towns in the evacuation zone around the plant 80 miles north of Kiev, capital of the Ukraine.

The ruling Politburo on Thursday discussed the ''intensive effort'' to decontaminate the surrounding area. It was the first report in a month of a Politburo discussion of Chernobyl.

Few details were given of the meeting, but the official news agency Tass sid: ''Measures were endorsed to ensure the employment, provision of housing and improvement of social and everyday services to the population evacuated from the danger zones.'' It said all evacuees should have jobs by the end of June and ''well-appointed housing'' by October.