The Russian Federal Security Service (FSB) has detained five buyers of a kilogram of radioactive Caesium-137 for $3.5 million on instructions from a "Ukrainian citizen," TASS reported on Friday, citing the FSB.

The FSB was cited as saying that the Caesium-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.

Ukraine has not responded to this claim.