Outgoing NATO Secretary General Anders Fogh Rasmussen has named Islamic State, or IS, and Russia as the key threats facing the military alliance, accusing Russia of "trampling" on post-Cold War peace.

Speaking Monday at a meeting of the Carnegie Europe think tank in Brussels, Rasmussen said NATO member states faced challenges in the east and the south "on a scale we have not seen over two decades, and they will endure for years to come."

"To the east, there is Russia. We have tried long and hard to build a partnership with Russia in a way that respects Russia's security concerns and based on international rules and norms," Rasmussen said, according to a video of the speech published online by NATO.

"Regrettably Russia has rejected our efforts to engage. Instead, Russia considers NATO — and the West more broadly — as an adversary. Russia has trampled all the rules and commitments that have kept peace in Europe and since the end of the Cold War," he added.

Rasmussen, who is stepping down at the end of the month after five years in the job, has been a vocal opponent of Russia's "reckless" policy on Ukraine, urging Moscow to drop its perceived support for pro-Russian separatists in the east of that country.

The 61-year-old Dane also warned against the new threat posed by Islamic militant group IS, which has taken over large swathes of territory in Syria and Iraq in recent months.

While ruling out NATO military intervention against the group, Rasmussen said the "horrific atrocities" carried out by IS could provide the basis for United Nations action in the Middle East.

The U.S. has already carried out several air strikes on IS forces in Iraq, though Russia warned last week it would consider any strikes carried out without a UN mandate over Syria — a Russian ally — as an act of aggression.