By Adrian Croft

BRUSSELS (Reuters) - Military intervention against Islamic State militants could be justified on the grounds of self-defense or preventing a campaign "pretty close to genocide", NATO Secretary-General Anders Fogh Rasmussen said on Monday.

Rasmussen said the threat posed by Islamic State "requires a military response to degrade and defeat this terrorist organization" but he said NATO as an organization would not be undertaking military strikes against the group.

"We are not considering ... a leading NATO role in this operation. A number of NATO allies are forming a coalition that also includes countries from the region," he told the Carnegie Europe thinktank where he gave his last speech in Brussels before stepping down at the end of this month.

The United States, NATO's dominant member, is already carrying out air strikes against Islamic State in Iraq.

Russia said last week that air strikes against Islamist militants in Syria without a U.N. Security Council mandate would be an act of aggression, raising the possibility of a new confrontation with the West in coming weeks.

Rasmussen said that, speaking as a "layman", he felt there was a basis in fundamental principles of the United Nations' charter to take military action against Islamic State.

"ISIS (Islamic State) commits horrific atrocities and I would say witnessing manslaughter, their attacks against religious and ethnic minorities, in my opinion it is pretty close to genocide. In my opinion that gives such a military operation legitimacy within the principles of the U.N. Charter," he said.

"I also consider this a kind of self-defense, which is also permitted within the U.N. charter," he said.

NATO agreed at its summit in Wales earlier this month to play a coordinating role in organizing security assistance for Iraq in its fight against Islamic State militants, including coordinating the airlift of supplies.

Rasmussen said the world should learn from previous military operations, such as Libya, that it was important to work after a military operation to help such countries improve their security and good governance.

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Since NATO-led air strikes in 2011 helped dislodge Muammar Gaddafi after 42 years of one-man rule, Libya has been unable to impose authority over brigades of former rebels who refuse to disarm and have carved out regional fiefdoms.

"That is one of the important lessons to be learned, that military operations should go hand in hand with civilian efforts to ... build a new nation after such a military operation," Rasmussen said.

(Reporting by Adrian Croft; editing by Ralph Boulton)