A former CIA director warned that the US and NATO allies would destroy Russian forces if Russian President Vladimir Putin used nuclear weapons in his war with Ukraine.

David Petraeus appeared on ABC's 'This Week' Sunday, where he said the nuclear threats by Putin need to be taken seriously, and that the ruler is 'desperate' as the 'battlefield reality he faces is irreversible.'

Putin signed a decree earlier this week to illegally annex parts of Ukraine. Days later, however, Russia was forced to withdraw troops from the strategically important Lyman – located in one of the four Ukrainian regions that Russia annexed.

While signing the decree, Putin also condemned the 'Satanic' West and vowed to use all of the Kremlin's might to protect Russia's new territory. The remark was widely seen as a nuclear threat. 

Petraeus said that, even though Ukraine is not a NATO country, a 'US and NATO response' would be in order. 

He told host Jonathan Karl that the likelihood that radiation from a nuclear weapon would extend to a NATO country could perhaps be construed as an attack on a NATO member. 

'This is so horrific that there has to be a response – it cannot go unanswered.' 

'Just to give you a hypothetical, we would respond by leading a NATO – a collective – effort that would take out every Russian conventional force that we can see and identify on the battlefield in and also in Crimea and every ship in the Black Sea.'