Separatist forces in Azerbaijan's breakaway region of Nagorny Karabakh on Sunday shot down two Azerbaijani military helicopters after Baku began bombing the enclave, rebel authorities said.
The worst clashes since 2016 have raised the spectre of a fresh large-scale war between arch-foes Azerbaijan and Armenia that have been locked for decades in a territorial dispute over Nagorny Karabakh.
On Sunday morning, Azerbaijan started "active bombing" along Karabakh's frontline and of civilian targets, including in the region's main city Stepanakert, Karabakh's presidency said. The rebel defence ministry claimed its troops shot down two Azerbaijani helicopters and three drones.
Azerbaijan's defence ministry said it launched a "counter offensive to suppress Armenia's combat activity and ensure the safety of the population."
"Let us stand firmly behind our state, our army... and we will win. Long live the glorious Armenian army!" Armenian Prime Minister Nikol Pashinyan wrote on Facebook.
Ethnic Armenian separatists seized Karabakh from Baku in a 1990s war that claimed 30,000 lives.
Talks to resolve the Karabakh dispute - one of the worst conflicts to emerge from the 1991 collapse of the Soviet Union - have been largely stalled since a 1994 ceasefire agreement.
France, Russia and the United States have mediated peace efforts as the "Minsk Group" but the last big push for a peace deal collapsed in 2010.
Energy-rich Azerbaijan has invested heavily in its military and repeatedly vowed to retake Karabakh by force.
Armenia has said it will defend the territory, which has declared its independence but relies heavily on Yerevan.