Armenia says that neighbouring Azerbaijan had attacked civilian settlements in the Nagorno-Karabakh region and urged the population in the region to seek refuge in shelters.
Armenia's Defence Ministry said its troops had downed two Azerbaijani helicopters and three drones in response to an attack against civilian settlements, including the regional capital of Stepanakert.
"Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation," the Armenian Defence Ministry said in a statement.
Azerbaijan's Defence Ministry, in turn, said it had launched a military operation along the "contact line," a heavily-mined no-man's-land that separates the Armenian-backed forces from Azeri troops in the region, Russian news agencies reported.
Azerbaijani President Ilham Aliyev gestures as he addresses the nation in Baku, Azerbaijan. Source: Azerbaijan's Presidential Press Office
The ministry said that an Azerbaijani helicopter had been downed but that its crew had survived.
Russia's Foreign Ministry called for an immediate ceasefire and urged both sides to hold talks to de-escalate tensions.
Over the past four decades, the two former Soviet countries have been in conflict over Nagorno-Karabakh, which is internationally recognised as part of Azerbaijan but controlled by ethnic Armenians.
Border clashes have intensified in recent months.
Armenia's Foreign Ministry condemned what it called the "aggression of the military-political leadership of Azerbaijan" and said the Armenian side would deliver an appropriate military and political response.
Ethnic Armenians in Nagorno-Karabakh declared independence during a conflict that broke out as the Soviet Union collapsed in 1991.
Though a ceasefire was agreed in 1994, Azerbaijan and Armenia frequently accuse each other of attacks around Nagorno-Karabakh and along the separate Azeri- Armenian frontier.
