NATO Secretary-General Anders Fogh Rasmussen says alliance members must come together to battle security threats both from Russia and Islamic State (IS) militants.
In a farewell speech as NATO leader at the Carnegie Europe think tank in Brussels on September 15, Rasmussen said the alliance must "stand strong as a force for freedom" and be prepared to use military force.
He added that NATO could play a role in coordinating military action against the IS, which he said was "a group of terrorists with whom there is no chance whatsoever to find any political solution."
Rasmussen, a former Danish prime minister, said that under President Vladimir Putin, Russia had "trampled all the rules and commitments" that had kept the peace in Europe since the end of the Cold War, and had conducted illegal military actions against Ukraine.
He said that Russia's long-term goal appears to be reestablishing "a zone of Russian influence in its near neighborhood and prevent neighboring counties from seeking Euro-Atlantic integration with NATO and the EU."
To achieve this goal, Rasmussen said, Russia would be interested in imposing a "new protracted frozen conflict" in Eastern Europe, along the lines of the existing frozen conflicts in regions such as Transdniester or North Caucasus.
He said: "The pattern is clear, from Moldova to Georgia and now in Ukraine, Russia has used economic pressure and military actions to produce instability, to manufacture conflicts, and to diminish the independence of its neighbors."
But he added that the NATO "should never accept a new frozen conflict in Eastern Europe."
Rasmussen, whose five-year term as NATO chief ends on September 30, said history teaches that "appeasement does not lead to peace" but rather "incites" tyrants.
He said NATO members must realize the new security challenges posed by Russia and IS could last for years.
Rasmussen expressed hope for continued enlargement of the alliance, saying that "Montenegro could be an important next step. That is why our door must and will stay open."
Based on reporting by Reuters and AP