Violence has flared up again in the conflict over the Nagorno-Karabakh region.
The Azerbaijani army announced on Wednesday that it had taken control of several strategically important areas in the disputed region.
According to both sides, an Azerbaijani soldier and two Armenian fighters had previously been killed in clashes.
According to Azerbaijan, fire had been opened on a position of its army in the district of Lachin, a buffer zone between the Armenian border and Nagorno-Karabakh.
A soldier was killed in the process.
As a result, a military operation called "revenge" was launched in response to "terrorist activities by illegal, armed Armenian groups".
However, the authorities in Nagorno-Karabakh accused Azerbaijan of violating an agreed ceasefire.
Two pro-Armenian separatists were killed and 14 others injured in an Azerbaijani drone strike.
Russia wants to “stabilize” the situation
The Armenian Foreign Ministry called on the international community to stop Azerbaijan's "aggressive actions".
Russia also accused Azerbaijan of breaking the ceasefire.
The Ministry of Defense in Moscow announced "measures to stabilize the situation".
The EU urged everyone involved to stop the violence immediately.
"It is crucial to de-escalate, fully respect the ceasefire and return to the negotiating table," said a spokesman for EU foreign policy chief Josep Borrell.
After an initial war in the 1990s, Armenia and Azerbaijan fought another war over the disputed Nagorno-Karabakh region in autumn 2020.
6,500 people were killed in the six-week fighting before the fighting was ended by a Russian-brokered ceasefire agreement.
Armenia had to give up large areas.
In mid-July, the foreign ministers of the two Caucasus states met for the first time since the war for bilateral talks in Tbilisi, Georgia.
The normalization of relations between neighboring countries is now being accompanied by the European Union.
Armenia's Prime Minister Nikol Pashinyan and Azerbaijan's President Ilham Aliyev met in Brussels in April and May.