An unexpected spike in radiation has been detected over Europe this week, and experts have no idea why.
An international body reported that sensors in Stockholm had picked up tiny amounts of unusual radioactive isotopes produced by nuclear fission.
The Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO), which monitors the world for evidence of nuclear weapons tests, said one of its stations scanning the air for radioactive particles had found unusual, although harmless, levels of caesium-134, caesium-137 and ruthenium-103.
The isotopes were "certainly nuclear fission products, most likely from a civil source", it said.
It tweeted a map showing where the material was likely to have originated, which included parts of several Baltic and Scandinavian countries as well as a swathe of western Russia. Asked about reports that Russia could have been the source of a leak, Kremlin spokesman Dmitry Peskov said: “We have an absolutely advanced radiation levels safety monitoring system and there are no any emergency alarms.
"We do not know the source of this information."
The International Atomic Energy Agency has asked countries whether they have detected the isotopes, and "if any event may have been associated with this”.
Finnish nuclear safety authority STUK said it had also found tiny amounts of nuclear particles in samples collected on its southern coast. But the concentrations were small enough that they could have been "derived from the normal operation or maintenance of nuclear reactors", it said. Radiation protection expert Jan Johansson at the Swedish Radiation Safety Authority said the variations were extremely low - far below levels seen in Sweden after the 2011 Fukushima accident far away in Japan - and had no impact on radiation protection.
"What stands out here is the combination of these substances. That's not something we usually see," he told Reuters.
The TASS news agency, citing Rosenergoatom, a unit of the state nuclear company Rosatom, said over the weekend that Russia's two northwest nuclear power plants, in Leningrad and Kola, were working normally and radiation levels were unchanged.