NATO will spend 3.0 billion euros ($3.9 billion) to buy and operate five US-built drones over 20 years in an effort to fill a gap exposed in the Libyan air war, an official said Wednesday.

Allies will pay at least 1.0 billion euros to acquire the Global Hawk drones from Northrop Grumman, a price that includes ground support stations, image analysis technology and training for operators, the official said.

Operating the drones, which will be based at the NATO base in Sigonella, Sicily, will cost the alliance another 2.0 billion euros over the next two decades, the official said on condition of anonymity.

"Libya showed the importance of having such a capability," the official said.

While European air forces carried out the bulk of bombing missions in Libya last year, they relied heavily on drones provided by the United States to identify and hit targets during the campaign.

NATO defence ministers finally agreed on the Alliance Ground Surveillance (AGS) after two decades of wrangling over its funding.

The drones are being purchased by 13 NATO nations: Bulgaria, Czech Republic, Estonia, Germany, Italy, Latvia, Lithuania, Luxembourg, Norway, Romania, Slovakia, Slovenia and the United States.

The aircraft will then be available to all 28 allies who will contribute to the cost of operating them. France and Britain will mostly contribute by providing their own surveillance aircraft to the programme.

NATO Secretary General Anders Fogh Rasmussen has hailed the AGS programme as a prime example of the alliance's efforts to pool and share resources at a time of economic crisis chipping away at defence budgets.