Azerbaijan said three of its troops were killed overnight when Armenian forces shelled its positions using mortars and grenade launchers, taking the overall death toll in the latest surge of violence to at least 36.
"In the event of continued Armenian provocations, we will launch a full-scale operation along the entire front line, using all kinds of weapons," Azerbaijan's defence ministry spokesman Vagif Dargahly told journalists.
The Armenia-backed separatist authorities in Karabakh -- which claims independence but is backed by Yerevan -- said that Azeri troops "intensified shelling of the Karabakh army positions on Monday morning, using 152-millimetre mortars, rocket-propelled artillery and tanks".
Azerbaijan claimed to have snatched several strategic positions inside the Armenian-controlled territory -- internationally recognised as part of Azerbaijan -- in what would be the first change in the front line since the ceasefire 22 years ago.
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In the Armenian capital of Yerevan, defence ministry spokesman Artsrun Hovhannisyan said Monday that Karabakh forces had "seriously advanced at certain sectors of the front line and took up new positions".
Russia and the West have called for a ceasefire, with President Vladimir Putin, a key power broker, pushing for an immediate end to the fighting, and Moscow's diplomats and military pressuring both sides.
"We are continuing contacts with Baku and Yerevan so that they hear the signals from Moscow, Washington and Paris," Foreign Minister Sergei Lavrov said Monday, mentioning the three capitals that have long mediated the conflict.
At least 18 Armenian, 15 Azeri troops and three civilians were reported killed, and one of Azerbaijan's attack helicopters was shot down.