Russian President Vladimir Putin said on Tuesday that Russia must defend itself if threatened, as the US warned against a return to "Cold War status" after Moscow announced plans to boost its nuclear arsenal.

The declaration came as Russia reacted with fury over reports the US is planning to bulk up its military deployments in Eastern Europe, adding to already high tensions between Russia and the West over the Ukraine conflict.

Putin accused NATO of "coming to our borders". The West has accused Russia of backing rebels in eastern Ukraine with weapons and its own troops, a claim Moscow denies.