Azerbaijan has said it has lost four troops in clashes with Armenia near the disputed Nagorno Karabakh region as mediators sounded the alarm over an increase in tensions in the protracted conflict.
The fresh confrontations on Saturday, which also led to the death of an ethnic Armenian soldier, come after Azerbaijan said on Friday that eight troops had been killed in three days of fighting.
Russia and the US have both expressed concern over the violence, the AFP news reported.
The two former Soviet nations have been locked in a long-simmering conflict over Nagorno Karabakh, a majority Armenian region within Azerbaijan that is de facto independent.
The sudden surge in tensions in a region that has been on a knife-edge for years comes as Armenia's ally Russia is locked in a confrontation with the West over the future of Ukraine.
Armenian-backed separatists seized Nagorno Karabakh from Azerbaijan in a 1990s war that claimed 30,000 lives.
A leading Azeri military expert said earlier this week that Baku has not suffered such losses in a single bout of hostilities since 1994, when a ceasefire was agreed.
The defence ministry in Baku said Saturday: "Armenia's reconnaissance and sabotage groups once again tried to attack Azeri positions at the line of contact" near Nagorno Karabakh.
Azeri troops repelled the overnight attack, forcing Armenian soldiers to retreat, the defence ministry said.
"As a result of the clash, four Azeri troops died," the ministry said in a statement.
Authorities in Nagorno Karabakh for their part said a 25-year-old ethnic Armenian soldier had been killed, and accused Azerbaijan of trying to carry out "sabotage and reconnaissance activities".
They claimed three Azeri troops had been killed and seven were wounded.
Black Sea talks
Armenia said weapons including grenades and mortars had been used against Nagorno Karabakh troops and that the Azeri troops' actions contradicted the negotiations under the auspices of the OSCE Minsk Group and threaten large-scale military hostilities.
Co-chaired by France, Russia and the US, the Minsk Group of the Organisation for Security and Co-operation in Europe has for years tried to broker a breakthrough in the conflict.
Later on Saturday, Armenia announced that President Serzh Sarkisian would meet with his Azerbaijani counterpart Ilham Aliyev in the Russian Black Sea resort of Sochi next week to discuss the crisis.
"The meeting between the two presidents will take place on August 8 or 9, in Sochi," Armenian Prime Minister Ovik Abraamian was quoted as saying by Russia's Interfax news agency.
Russia called the clashes a "serious violation of a ceasefire agreement and stated intentions to reach a settlement through political means".
"A further escalation is unacceptable," the Russian foreign ministry said.
Deputy US State Department spokeswoman Marie Harf said in a statement: "Retaliation and further violence will only make it more difficult to bring about a peaceful settlement."
Azerbaijan has threatened to retake Nagorno Karabakh by force if negotiations do not yield results, while Moscow-allied Armenia has vowed to retaliate against any military action, AFP said.
Agasi Yenokyan, a political analyst in Armenia, suggested that the clashes may be in the interests of Russia, which he claimed wants to station troops in Nagorno Karabakh to boost its presence in the Caucasus.
"Armenian authorities have several times hinted they are capable of preserving peace in the region themselves, without the help of third parties," he said.