Nato leaders are gathering in the Spanish capital Madrid for a "historic and transformative" summit in which the alliance will hammer out its new strategic concept to face its most serious security crisis since World War II, secretary-general Jens Stoltenberg said Wednesday.

Allied heads of state and government are expected to state in the strategic concept, Nato's security roadmap for the next decade, that Russia poses a "direct threat" to Nato's security, the Norwegian politician told reporters upon his arrival at the summit.

"We meet in the midst of the most serious security crisis we have faced since the Second World War," Stoltenberg said. "And we see that Allies are able to demonstrate unity."