Armenia alleged a Turkish jet of shooting down one of its fighter jets in a major escalation of the conflict over the disputed Nagorno-Karabakh region.
The Armenian foreign ministry said the pilot of the Soviet-made SU-25 died after being hit by the Turkish F-16 in Armenian air space.
Turkey, which is backing Azerbaijan in the conflict, has denied the claim.
Nearly 100 people, including civilians, have died in three days of fighting over the disputed mountainous region.
The enclave is internationally recognised as part of Azerbaijan, but has been run by ethnic Armenians since a 1988-94 war between the two former Soviet republics.
Azerbaijan has repeatedly stated that its air force does not have F-16 fighter jets. However, Turkey does.
The fighting that started three days ago now appears to be spilling out of Nagorno-Karabakh, with Armenia and Azerbaijan trading accusations of direct fire into their territories.
They also blame each other for starting the conflict.
While Turkey openly backs Azerbaijan, Russia – which has a military base in Armenia but is also friendly with Azerbaijan – has called for an immediate ceasefire.
On Tuesday, both Armenia and Azerbaijan said that heavy fighting had continued overnight in Nagorno-Karabakh.
The region’s self-proclaimed authorities said 87 of their military personnel had been killed and 120 wounded since the fighting began on Sunday, according to the Armenpress news agency.
They put the number of deaths on the Azerbaijani side at nearly 400, saying that one aircraft, four helicopters and a number of tanks had been destroyed.
Azerbaijan has released no figures on its military casualties, but says 12 civilians were killed by Armenian fire.
Azerbaijan’s defence ministry was quoted by news agencies as saying that Armenian troops had repeatedly tried and failed to regain lost positions in the Fuzuli-Jabrayil and Aghdere-Terter areas.
The ministry said that a column of Armenian armoured and other combat vehicles had been destroyed, adding that the enemy had suffered heavy losses.
The casualty claims by Armenia and Azerbaijan have not been independently verified.
How it started?
Fighting erupted between Armenia and Azerbaijan over the disputed Nagorno-Karabakh region on Sunday.
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 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.
Both nations broke away in the dying years of the Soviet Union. They were part of the communist state, which sought to suppress ethnic and religious differences.