nuclear weapon

Putin has threatened the use of nuclear weapons before, and Ukraine has criticised it as "nuclear sabre-rattling" to deter its allies from providing further support.

At times, Putin has mooted the possibility of using nuclear weapons, too.

The following month, the Kremlin leader announced imminent changes to the Russian nuclear doctrine, the document setting out the preconditions under which Moscow might decide to use a nuclear weapon.

It was like being "microwaved", he tells the BBC film, as his body was exposed to the raw power of a nuclear weapon.

Trump has likened the new initiative to the Manhattan Project, a top-secret World War Two programme to develop the first nuclear weapons.