The FBI has uncovered an alleged plot by smugglers to sell deadly radioactive substances to ISIS jihadis.

In the past five years, four separate conspiracies have been thwarted involving groups with Russian connections looking to make shadowy deals with Middle Eastern terror groups.

In February a smuggler was allegedly looking for an Islamic State buyer for his stock of deadly caesium - large enough to poison hundreds of homes and offices.

The operation was based in Moldova where criminal organisations, some with links to the Russian KGB's successor agency, are involved in black market of nuclear materials.

In 2011, it found a group led by a Russian known only as 'The Colonel' which attempted to arrange for the sale of bomb-grade uranium and blueprints for a dirty bomb to a man from Sudan. The plot was uncovered, but the gang leader escaped.

A middleman reportedly told a spy he was keen for a Middle Eastern terrorist to make the deal because they would bomb Americans.

Russia has a vast stock of radioactive material, some of which has seeped into the black market.