European countries, who take the lead in NATO's Libya mission, could not get the job done without vital military capabilities provided by the United States, the alliance's chief Anders Fogh Rasmussen said on Monday.

"European Allies and Canada led the effort. But this mission could not have been done without capabilities which only the United States can offer. For example: drones, intelligence and refuelling aircraft," Rasmussen told reporters.

"Let me put it bluntly: those capabilities are vital for all of us. More Allies should be willing to obtain them. That is a real challenge. And we will have to find the solutions at the next NATO Summit in Chicago," he said.

The NATO chief said that "the Libya operation has demonstrated that despite the periods of economic austerity, despite defense cuts in a number of countries, maybe all countries, it was possible for European allies and Canada to provide the majority of assets for our Libya mission, strike aircraft and also maritime assets."

"But the whole operation has made it visible that there are some gaps (between European countries and the United States) to be filled," he said.

Rasmussen's remarks came around three months after former U.S. Defense Secretary Robert Gates sharply criticized some European nations' reluctance to expand defense budgets and unwillingness to take on combat missions, which has created a two-tier alliance: the United States at one level and the rest of NATO members on a lower, almost irrelevant plane.

Gates said NATO's operations in Afghanitan and Libya had exposed shortcomings of the military alliance, particularly lack of military capabilities and political will.