Armenia and Azerbaijan report heavy fighting around the separatist Nagorno-Karabakh region Sunday.
Armenia accused neighboring Azerbaijan of attacks on civilian settlements in the disputed region.
Armenia's Defense Ministry said its troops shot down two Azerbaijani military helicopters and three drones after Baku’s forces began bombing the breakaway enclave, including its capital Stepanakert.
"Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation," the Armenian Defense Ministry said in a statement.
However, Azerbaijan's Defense Ministry said it had launched a military operation along the "contact line," between the two countries, “to suppress Armenia's combat activity and ensure the safety of the population."
The ministry confirmed the downing of only one Azerbaijani helicopter and said its crew had survived.
Armenian separatists seized Nagorno-Karabakh from Azerbaijan in a bloody war in the 1990s that killed 30,000 people.
Talks to resolve the conflict about the predominantly Armenian populated enclave have been halted since a 1994 cease-fire agreement between Armenia and Azerbaijan.
The peace efforts mediated by France, Russia and the United States as the "Minsk Group" collapsed in 2010.