Turkey, meanwhile, urged the alliance to call for the withdrawal of Armenian forces from the region, which belongs to Azerbaijan under international law but is populated and governed by ethnic Armenians.
In the ninth day of fighting, Armenia and Azerbaijan accused each other on Monday of attacking civilian areas, where hundreds of people have been killed in the fiercest clashes in the region for more than 25 years.
Speaking alongside Turkish Foreign Minister Mevlut Cavusoglu in Ankara, Stoltenberg said there was no military solution to the conflict over Nagorno-Karabakh.
“It is extremely important that we convey a very clear message to all parties that they should cease fighting immediately, that we should support all efforts to find a peaceful, negotiated solution,” Stoltenberg said.
Turkey has condemned what it says is Armenian occupation in Nagorno-Karabakh and vowed full solidarity with ethnic Turkic Azerbaijan. Cavusoglu said NATO should also call for the withdrawal of Armenian forces from the region.
“Azerbaijan is battling in its own lands, it is trying to take back its lands from terrorists and occupiers. Legally and morally, everyone should support Azerbaijan in that sense,” Cavusoglu said.
“Everyone, namely NATO, should call for the resolution of this problem under international laws, U.N. resolutions and Azerbaijan’s territorial integrity, so by Armenia immediately withdrawing from this region.”
The fighting began on 27 Septtember and has surged to its worst level since the 1990s, when some 30,000 people were killed.
In a video address to commanders, Turkish Defence Minister Hulusi Akar said Armenia was targeting civilians, and it “must immediately withdraw from the lands it occupies without committing any further humanitarian crimes”.