Russia's aggression against Ukraine is not an isolated incident, but a game-changer in European security, said NATO Deputy Secretary General Alexander Vershbow.

"It reflects an evolving pattern of behavior that has been emerging for several years, despite our efforts to reach out to Russia and build a cooperative European security system with Russia," he said giving a speech to the Nobel Institute in Oslo on Monday.

Vershbow called on Russia to de-escalate the situation and to stop sending weapons and supplies to the rebels and to pull its soldiers and advisors out of Eastern Ukraine.

He said Russia cannot select the rules it wants to obey and ignore the others.

"We must stay united, stay firm and increase the costs to Russia of its aggression. Over time, Russia will see that it is in its own best interests to return to a policy of cooperation – but only if we show it, that we take our principles seriously."