The clashes have spurred a flurry of diplomacy to defuse the reignited tensions between majority Christian Armenia and mainly Muslim Azerbaijan. Russia called for an immediate ceasefire and another regional power, Turkey, said it would support Azerbaijan, its traditional ally.
Armenia's ambassador to Russia said on Monday Turkey had sent around 4,000 fighters from northern Syria to Azerbaijan, Interfax news agency reported, an accusation denied by Baku.
Under international law, Nagorno-Karabakh is recognised as part of Azerbaijan. But the ethnic Armenians who make up the vast majority of the population reject Azeri rule.
They have run their own affairs, with support from Armenia, since Nagorno-Karabakh broke away from Azerbaijan in a conflict that erupted as the Soviet Union collapsed in 1991.Although a ceasefire was agreed in 1994, after thousands of people were killed and many more displaced, Azerbaijan and Armenia frequently accuse each other of attacks around Nagorno-Karabakh and along the separate Azeri-Armenian frontier.