In a speech kicking off the 40-nation civil emergency drills, Stoltenberg acknowledged lingering public unease over NATO, which conducted an 11-week bombing campaign in Serbia nearly two decades ago to force the end of a bloody war between Serb troops and Kosovar separatists.

"I realise that NATO remains controversial in Serbia. The memories of the air campaign in 1999 are still painful for many," Stoltenberg said in a press conference with Serbian President Aleksandar Vucic.

"We must never forget the past. But we can move beyond it. And that is what NATO and Serbia are doing with our partnership," he added.

Unlike neighbouring Montenegro, which joined NATO last year, Serbia has chosen to remain outside the formal alliance, maintaining its close ties with Russia.

Stoltenberg said NATO "fully respects" Serbia's decision not to join the alliance.

According to Belgrade Centre for Security Policy, Serbia has participated with NATO members in some 150 exercises since it joined the Partnership For Peace programme in 2006.

In last five years Serbia also took part in a dozen joint exercises with Russia, according to the same source.

Vucic said Serbia was satisfied with cooperation with NATO "on the rational level", but added the alliance remained unpopular in the Balkan country because of "emotional and psychological issues".

The civilian death toll from the 1999 bombing has never been officially established and figures vary from 2,500 dead claimed by Serbian officials to 500 in a Human Rights Watch estimate.