Sixteen Azeri servicemen have been killed in fighting around the Armenian-backed breakaway Nagorno-Karabakh region in the past two days, Azerbaijan's Defence Ministry said on Tuesday, as a flare-up of a decades-old conflict showed no sign of abating.
Ex-Soviet states Azerbaijan and Armenia fought a war over the mountainous territory in the early 1990s in which thousands were killed on both sides and hundreds of thousands displaced.
The war ended with a fragile truce in 1994, followed since by sporadic bouts of violence. The ceasefire was shattered over the weekend with the fiercest fighting in years, killing dozens of people on both sides.
Azerbaijan and Narogno-Karabakh, which has enjoyed strong military and financial support from Armenia, accused each other on Tuesday of escalating the violence.
"All responsibility for what is happening rests with Armenia which is not interested in resolving the conflict and flouts international law," Azeri Deputy Foreign Minister Khalaf Khalafov told a regional meeting in the Azeri capital Baku.
The clashes, involving tanks, helicopters and artillery, are around the contact line, a heavily-mined no-man's land that has since 1994 separated the Armenian-backed forces, in the foothills of the Karabakh mountains, from Azeri troops dug into defensive positions in the plains below.
In a statement, the armed forces of Armenian-backed Nagorno-Karabakh said Azerbaijan "had been increasing the caliber of its weapons day by day", and had used Smerch multiple-launch rocket systems on Monday night, shelling civilian settlements and military strongholds.
They did not say if they had suffered fresh losses.
A return to war would destabilize a region that is a crossroads for strategically-important oil and gas pipelines.
It could also drag in the big regional powers, Russia and Turkey. Moscow has a defense alliance with Armenia, while Ankara backs Azerbaijan.
Envoys from France, Russia and the United States, joint mediators in the conflict, are to meet in Vienna on Tuesday for talks on the fighting.
Nagorno-Karabakh is a mountainous enclave with a large ethnic Armenian population that lies inside the territory of Azerbaijan. The violence was a re-awakening of a long-festering ethnic conflict between the mainly Muslim Azeris and their Christian Armenian neighbors.