Russia's FSB security service said on Friday it had detained five people who were trying to buy 1 kg of radioactive Cesium-137 for $3.5 million on behalf of a citizen of Ukraine, the TASS news agency reported.
The FSB was cited as saying that the Cesium-137 was meant to be taken out of Russia to be used to stage an incident with purported weapons of mass destruction aimed at discrediting Russia.
Reuters could not independently confirm the assertions and there was no immediate comment from Ukraine.
Back and forth accusations
Ukrainian President Volodymyr Zelensky said this week that Ukrainian spies had received information showing Russia was considering carrying out a "terrorist" attack at the Russian-controlled Zaporizhzhia nuclear plant in southern Ukraine involving a release of radiation.
Russia has denied that and in turn accused Kyiv of planning to seize the plant and of shelling its facilities including those where nuclear fuel is stored. It has also accused Kyiv of planning a "dirty bomb" attack against Russia, something Ukraine has denied.
The FSB said on Friday that the people it had detained in the Cesium -137 case were already confessing. It provided no other details such as their names or backgrounds.