Karis said that while very few people actually know what Putin thinks, he is definitely not insane in medical terms. "That means he knows exactly what he is doing," the Estonian president said.

He said that while he is not too concerned about the likelihood of Putin using nuclear weapons, an eye needs to be kept on the possibility should the situation become "very desperate" for Russia. "But it's not that easy. It's not that you have a button in the corner, and then you go and push it. There are still certain steps to follow," Karis added.

Concerning Putin's successor, the Estonian president said it may be a while before we see a Russian leader willing to work with the rest of the world.

Karis said a large question mark hangs over what the Russian people want. "You can't get proper information on what Russian people actually think, because they have been silenced, like being under the Soviet regime," he said.

The president also said that he cannot see a chaotic breakup of Russia occurring should Ukraine end in a comprehensive defeat.

"It's difficult to expect. The resources are not in Moscow or St. Petersburg, they're in Siberia," he said, noting that no successor state could afford to lose access to such lucrative assets. "It won't happen, it simply won't happen."

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