Russia has accused Ukraine of preparing to use such a weapon against Moscow's troops, but Kyiv suspects Russia might itself use a dirty bomb in a "false flag" attack, possibly to justify use of conventional nuclear weapons by Moscow as it finds itself on the back foot in eastern and southern Ukraine.

International Atomic Energy Agency (IAEA) head Rafael Grossi said: "IAEA inspectors will conduct independent verification at these locations".

He said they would work "to detect any diversion of nuclear material under safeguards, any undeclared production or processing of nuclear material at the two locations and assure that there are no undeclared nuclear material and activities".

The agency said it had inspected "one of the two locations a month ago and no undeclared nuclear activities or materials were found there."

A dirty bomb is a conventional bomb laced with radioactive, biological or chemical materials which are spread in an explosion.

Earlier on Thursday Russian President Vladimir Putin called for the IAEA to inspect Ukraine's nuclear sites "as fast as possible".