Russia's actions have been a "game changer" and investment in defense needs to be increased, secretary general of Nato Anders Fogh Rasmussen told CNBC.
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"Now we must reverse the trend of cuts and gradually increase defense spending," said Rasmussen.
"During the last five years, Russia has increased defense spending by 50 percent, while NATO allies on average have decreased their defense spending by 20 percent. That is not sustainable, we need more investment in defence and security," he added.
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Recent meetings with political leaders such as German Chancellor Angela Merkel and U.K. Prime Minister David Cameron revealed "grave concern" about the "dramatically changed security situation" in Europe, the NATO chief said.
He warned security can no longer be "taken for granted" in the wake of the Ukraine crisis, but also as tensions have re-emerged in the Middle East and North Africa.
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"We have to ensure effective protection against any kind of threat, conventional threats as well as emerging security challenges like possible missile attack sand cyber attacks," he said.
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