NATO Secretary General Jens Stoltenberg has accused Russia of prolonging the war in Syria by supporting President Bashar Assad and calls on Moscow to join the fight against Islamic State militants.

Stoltenberg says Russia's violation of Turkish and NATO airspace is unacceptable and that the alliance is worried about Russia's "substantial military build-up, its air strikes and its cruise missile attacks" in the Middle East.

Noting that many countries in the region and NATO allies are participating in the U.S.-led mission against the militants, he called on Moscow to "play a constructive role in the fight against ISIL." He adds that "supporting the Assad regime is not constructive ... (and) is only prolonging the war."

Stoltenberg spoke Monday at NATO's parliamentary assembly in the southwestern Norwegian port of Stavanger.