Fighting erupted between Armenia and Azerbaijan over the disputed Nagorno-Karabakh region, with both civilians and combatants killed.
Accusing Azerbaijan of air and artillery attacks, Armenia reported downing helicopters and destroying tanks, and declared martial law.
Azerbaijan said it had begun a counter-offensive in response to shelling.
Armenia’s defence ministry said an attack on civilian settlements in Nagorno-Karabakh, including the regional capital Stepanakert, began at 08:10 local time on Sunday.
A woman and child were killed, officials said. The separatist authorities in Nagorno-Karabakh said 16 of their servicemen had died, with 100 injured.
Armenia said it had shot down two helicopters and three drones, as well as destroying three tanks.
The region is internationally recognised as part of Azerbaijan but controlled by ethnic Armenians.
They broke away in the dying years of the Soviet Union. Both Armenia and Azerbaijan were part of the communist state, which sought to suppress ethnic and religious differences.
Amid the clashes, Azerbaijani President Ilham Aliyev said he was confident of regaining control over the breakaway region.
Martial law has also been declared in some regions of Azerbaijan.
The conflict in the Caucasus Mountains has remained unresolved for more than three decades, with periodic bouts of fighting. Border clashes in July killed at least 16 people, prompting the largest demonstration for years in the Azerbaijani capital Baku, where there were calls for the region’s recapture.
On Sunday, Turkey’s President Recep Tayyip Erdogan pledged support for Azerbaijan during the new crisis while Russia, traditionally seen as an ally of Armenia, called for an immediate ceasefire and talks to stabilise the situation.
“Get ready to defend our sacred homeland,” Armenian Prime Minister Nikol Pashinyan said after accusing Azerbaijan of “pre-planned aggression”.
France, which has a large Armenian community, called for an immediate ceasefire and dialogue, while Iran, which borders both Azerbaijan and Armenia, offered to broker peace talks.