VIENNA, Austria (AP) _ An American doctor who treated victims of the Chernobyl nuclear accident said Thursday the cancer death toll worldwide could in time reach 75,000, but an Argentine expert said many projections of a high death toll were ''nonsense.''

A Soviet official also told reporters that initial estimates of as many as 25,000 deaths in the European part of the Soviet Union alone had been based on the most pessimistic data available, and were 10 times too high.

The nuclear and medical experts spoke at a news conference on the fourth day of a week-long conference held under the auspices of the International Atomic Energy Agency called to consider ways to improve international cooperation in fighting nuclear accidents.

An explosion and fire occurred April 26 at the No. 4 reactor of the Chernobyl nuclear plant, 80 miles north of Kiev in the Soviet Ukraine, sending a radioactive cloud over much of Europe and other parts of the world. At least 31 people have died from the accident.

Dr. Robert Gale, a Los Angeles bone marrow surgeon who treated some Chernobyl victims, told reporters that health experts believe the number of cancer deaths worldwide resulting from the accident could range from 1,000 to 75,000 over the next 70 years.

Most conference delegates agreed as many as 25,000 cancer deaths linked to Chernobyl could occur in the European part of the Soviet Union alone, he said.

Gale told The Associated Press on Thursday that estimates of the Chernobyl death toll are ''very broad,'' but health experts believe ''the truth will lie between the extremes.''

He said there could be 9.5 million cancer cases in the western Soviet Union in the next 70 years even without Chernobyl. ''So lower figures would not be mind-boggling,'' but significantly higher death rates ''would be truly horrible,'' he said.

Dan Beninson, an Argentine nuclear safety official, predicted 2,000 cancer deaths in the Soviet Union from Chernobyl. Estimates in the 20,000 range are ''nonsense, in many respects,'' he told reporters.

''You can have fun with numbers but you must be very careful,'' Beninson said.

''That does not mean that if we have only 2,000 instead of 20,000 it is good or more acceptable,'' he added. ''It is a very bad number in any case. But it has to be put into perspective.''

Other delegates also complained that public predictions of cancer cases caused by Chernobyl tend to be exaggerated because they rely on worst-possible scenarios, and may be 10 times too high.

They also said the calculations contained in a technical report the Soviets presented to the conference this week had been based on pessimistic theoretical calculations rather than actual measurements.

Leonid Ilyin, vice president of the Soviet Academy of Medical Sciences, told reporters the initial Soviet estimates, which have not been made public, were based on a calculation of worst possible exposure to Chernobyl radiation.

''We have carried out a wide campaign of measuring real levels (of exposure to the 135,000 people evacuated from the 18-mile zone around Chernobyl) and these real levels turned out to be 10 times less than the calculated values,'' Ilyin said.

''That's because our measuring standards are based on the maximum possible effect of radiation release,'' said Ilyin. ''This allows us to calculate that malignant tumors from Chernobyl will be considerably less than 1 percent of existing cancers in the population.

Ilyin said Sovie doctors continued to monitor the radiation exposure of the Chernobyl evacuees.

''In light of the forecast dose burden we now have for these people, there is no fear that any special measures will have to be taken by the medical services,'' said Ilyin, whos remarks in Russian were simultaneously translated into English.

''The effect will be insubstantial,'' he said. ''We have come to a unanimous conclusion on that.''

Helmut Rabold, an East German atomic safety official, said Soviet and Western delegates had suggested establishing an international agency, composed of civil defense and military personnel, to pool efforts against any future atomic accident.