Fighting between Armenia and Azerbaijan broke out on Sunday over the region of Nagorno-Karabakh.
At least 32 separatist soldiers from the Armenia-backed region had been killed in the flare-up of violence since it started on Sunday morning, AFP on Monday cited defence ministry officials in Nagorno-Karabakh as saying.
The President of Azerbaijan said in a televised address that there have been "casualties among the civilian population and our servicemen" but did not announce any number for military losses.
However, the loss of life could be high, with both sides claiming to have inflicted hundreds of casualties on the other.
What has the international reaction been?
European Council President Charles Michel tweeted that the fighting was of "serious concern."
"Military action must stop, as a matter of urgency, to prevent a further escalation," Michel said.
Turkey’s ruling party spokesman Omer Celik tweeted: “We vehemently condemn Armenia’s attack on Azerbaijan. Armenia has once against committed a provocation, ignoring law.”
He said Turkey, which has close ties with the predominantly Turkic Azerbaijanis, would stand by Azerbaijan, adding: “Armenia is playing with fire and endangering regional peace.”
Russian President Vladimir Putin spoke on the phone with Armenia's prime minister and expressed "grave concern" over the hostilities, the Kremlin said in a statement.
Russia has long been seen as an ally of Armenia.
Albanian Prime Minister Edi Rama, who is chairperson of the Organisation for Security and Cooperation in Europe (OSCE), called for a de-escalation of the situation.
“I urge all involved to immediately return to the ceasefire before the human toll of this conflict increases any further,” Rama said.
The OSCE has historically worked to broker peaceful negotiation between the countries.
A history of conflict
The former Soviet states of Azerbaijan and Armenia fought a bloody war over the mountainous region of Nagorno-Karabakh in the early 1990s.
Thousands were killed on both sides. Hundreds of thousands of people were displaced.
The war ended with a truce in 1994, although there has been sporadic violence since.