Any encroachment on the Crimea by NATO could be equal to a declaration of war on Russia which could lead to "World War Three," Russia's former president, Dmitry Medvedev, was quoted as saying on Monday.

"For us, Crimea is a part of Russia. And that means forever. Any attempt to encroach on Crimea is a declaration of war against our country," Medvedev told the news website Argumenty i Fakty.

"And if this is done by a NATO member-state, this means conflict with the whole alliance,; a World War Three. A complete catastrophe."The NATO Madrid summit will begin today.