You used the word “prefer”. That's the right word. NATO will always look to the UN for a mandate for expeditionary operations. Except for a moment of the Kosovo operation, that has always been the case. NATO does not need the mandate of the United Nations to operate. Certainly, when it comes to the defence of an ally, it has a treaty obligation to each other, which does not require UN mandates. When we are defending ourselves, we can do that on our own authority.
On May 1st, 2012. See this statement in context.