Azerbaijan’s Defence Ministry said it launched a “counter attack” after accusing Armenians of firing on its military positions and on civilian settlements near the disputed Nagorno-Karabakh territory.
Armenia said Azerbaijan started bombarding along the contact line separating the two forces and shelled civilians, including in the region’s capital, Stepanakert.
The conflict over Nagorno-Karabakh has flared repeatedly since Armenians took control of the territory and seven surrounding districts from Azerbaijan in a war after the Soviet Union collapsed in 1991.
Hundreds were killed in a five-day war in 2016 before Russia pressured both sides to resume a ceasefire that was first brokered in 1994.
Despite decades of mediation by the US, France and Russia, no peace agreement has been signed.
The Azeri army is using tanks, artillery, missile systems and aircraft against Armenian positions near the front line and deeper into Armenian-held territory, the Defence Ministry in Baku said in a website statement.
As many as 12 Armenian anti-aircraft systems have been destroyed and one Azeri helicopter was shot down, it said.
Armenian forces hit two Azeri helicopters, three tanks and three drones, Armenian Defence Ministry spokesman Shushan Stepanyan said.
Civilians in Nagorno-Karabah have been urged to go into shelters after Azerbaijan shelled Stepanakert, according to Mr Vahram Poghosyan, presidential spokesman in the unrecognised republic.
“Our response will be proportional, and the whole responsibility is on the military-political leadership of Azerbaijan,” he said.
The fighting comes after tensions between the two sides rose sharply last week, when Azeri President Ilham Aliyev alleged that Armenia was massing forces near Nagorno-Karabakh for a new war.
Armenia dismissed the claim as groundless and accused Mr Aliyev of preparing for a war.