MOSCOW—Russia said Tuesday for the first time that it had detected a significant radiation spike in the Ural Mountains, close to a sprawling Soviet-era nuclear plant still remembered as the site of an accident 60 years ago. But it rejected suggestions that it was the source of a radioactive cloud that hovered over Europe.

The location of the spike — in the Chelyabinsk region near the border with Kazakhstan — has been identified by French and German nuclear safety institution as a potential source for a concentration of a radioactive isotope called ruthenium 106 detected in the air in late September above several European countries.

But nuclear energy authorities in Moscow insisted Monday that still-higher levels of atmospheric contamination had been detected outside Russia, in southeastern Europe.

Reports of the elevated radiation levels over Western Europe raised alarms, but nuclear safety authorities in France and Germany said there was no threat to human health or to the environment — an assurance repeated Tuesday by Moscow.

The Russian state weather service Roshydromet said it had found what the Russian news media described as “extremely high pollution” at two monitoring facilities within a 100-kilometre radius of the Mayak nuclear reprocessing and isotope production plant.

A weather station in the town of Argayash recorded ruthenium 106 levels that were 986 times higher than a month earlier, the state weather agency said. A second station at Novogorny detected levels 440 times higher. Ruthenium 106, which does not occur naturally and has a half life of about a year, is used for medical purposes.

For weeks, Russian officials had denied the French and German accusations. Citing the results of its own air monitoring on European territory, Moscow pointed to high radiation levels over Romania, Italy and Ukraine, insisting that there had been only a negligible presence of ruthenium 106 on Russian territory.

On Tuesday, even after Russia acknowledged the radiation spike in the Urals, Maxim Yakovenko, the head of Roshydromet, said in a statement that higher levels of contamination had been detected in Romania than in Russia. “The published data is not sufficient to establish the location of the pollution source,” he said.

During the Cold War, Mayak was part of a string of closed military areas whose existence and function were supposed to be kept secret. The accident in 1957, which released radioactive waste, was kept under wraps for decades.

The authorities at Mayak denied in a news release Tuesday that the plant had contributed to the increased levels of ruthenium 106 and insisted that there was no threat to humans.

“The dose which a person could be subjected to, according to the Roshydromet data, is 20,000 times lower than the safe yearly dose, and does not represent any danger whatsoever to people’s health or their lives,” the news release said.

Rosatom, the state company that runs Russia’s nuclear industry, said there had been no accidents at its plants that could explain the concentration of ruthenium 106. “Rosatom categorically confirms there have been no unreported accidents or reportable events on any of its nuclear sites,” the company said. “The highest concentrations are in areas outside Russia.”

It said that “relatively high levels” of ruthenium 106 had been detected “in the air in some parts of Russia.”

“None of the enterprises of the Russian nuclear industry have recorded radiation levels that exceed the norm,” Rosatom said. The nuclear authorities in Kazakhstan also denied responsibility for the pollution.

The vast Mayak facility once produced plutonium for Soviet nuclear weapons. In the 1957 breach, a nuclear waste storage tank exploded, sending a radioactive cloud over an area estimated at 20,000 square miles.

The accident is considered to be one of the worst on record after those at Chernobyl in Ukraine in 1986 and Fukushima, Japan, in 2011. A further accident was reported at Mayak in 2004 when officials confirmed that waste had been dumped in a nearby river.

The memory of Mayak has fed skepticism among Russians about the state’s readiness to disclose what is really going on now. Some have taken to social media to voice their frustration. “1986 — silence over Chernobyl … 2017 — silence over Chelyabinsk,” a user named SivSA posted on Twitter.

Officials in the Mayak region said there were no plans to evacuate the region for now. “First we need to figure this all out,” the Chelyabinsk region’s vice governor, Sergey Sushkov, said.

For their part, Russian scientists have called for calm.

Ilia V. Yarmoshenko of the Institute of Industrial Ecology in Yekaterinburg, which monitors radiation in the Urals, said there was no cause for concern. Radiation levels in an average apartment in the South Urals are “10 times higher” than the levels recorded by Roshydromet, he told Znak.

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The report by Roshydronet was compiled in September, but was released only on Tuesday in response to a request for information by Greenpeace. On Tuesday, Greenpeace said it would petition the Russian state prosecutor’s office to investigate “a possible concealment of a radiation incident.”

The Mayak plant was also mentioned in inquiries into the poisoning death of Alexander Litvinenko, a KGB defector poisoned with radioactive polonium 210 in London in 2006. Noman Dombey, an emeritus professor of theoretical physics at the University of Sussex, told a public inquiry in 2015 that the polonium used to poison him could only have come from Russia because production had ceased in other places.

Dombey said that polonium was produced at another plant, using bismuth irradiated at Mayak. In early 2016, the inquiry found that President Vladimir Putin of Russia had probably approved the Litvinenko poisoning.

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