Hostilities between Armenia and Azerbaijan have reportedly flared up overnight, with reports of gun and mortar fire coming from the border. Moscow has called for an end to hostilities. Both Azeri and Armenian defense ministries accused each other of provoking the escalation. Azerbaijan said Armenian troops opened fire 127 times over 24 hours along the border. They are using mortars and heavy machine guns, the statement released on Saturday said.

Armenia said Azeri troops went on the offensive overnight and are using tanks artillery and military aircraft. Militias in the unrecognized Karabakh republic, the focal point of the conflict, claimed they shot down an Azeri helicopter gunboat. Azerbaijan has denied those reports. Russian President Vladimir Putin has called on all sides in the conflict to immediately cease hostilities, the Kremlin spokesman said.

The two former soviet republics are locked in a decades-long conflict over Karabakh, a predominantly Armenian mountainous region that was part of Azerbaijan, but broke away in 1988. The region declared independence in 1991, with a bloody three-year war following it. Russia brokered a ceasefire between Armenia and Azerbaijan in 1994, but the tensions have never actually stopped since then and there is occasional violence.

The mutual distrust between Armenia and Azerbaijan is rooted in a long history of ethnic and religious conflicts in the region as well as their participation in the rivalry of regional heavyweights – the Turks, the Russians and the Persians over the centuries. Both nations had their first bid for statehood in the wake of the collapse of the Russian Empire in the early 20th century. Among other things their independence resulted in a war in 1918. When Moscow reinstated its control over the region, the conflict was swept under the rug, but never fully extinguished.

The current escalation comes amid the standoff between Russia and Turkey over the Syrian conflict. Ankara supports Azerbaijan, while Armenia is an ally of Moscow.