The territorial dispute has its origins in the disintegration of the Soviet Union, when Nagorno Karabakh, encouraged by Armenia, declared independence from Azerbaijan, triggering a violent conflict which ended in the 1994 ceasefire. Since 1994, there have been episodic clashes with 300 border incidents in the past five years alone.
Long-running talks overseen by France, the US and Russia as co-chairs of the Organization for Security and Co-operation in Europe, OSCE, Minsk Group have stalemated.
Fighting between Azerbaijani and Armenian forces, including Nagorno-Karabakh fighters, erupted on 27 September after months of smaller border clashes. The territory, which is crisscrossed by energy pipelines, is of strategic importance to Russia, Turkey and Iran.
While Armenia has said it is ready to discuss a ceasefire, Azerbaijani President Ilham Aliyev has so far shrugged off talk of a truce and appears determined to press on with a military campaign involving heavy artillery, tanks and drones to wrest control of Nagorno-Karabakh and seven other Armenian-controlled territories adjacent to the enclave.
Some analysts say Azerbaijan has been emboldened by its ally Turkey and fear this hardline position will make it much more difficult to negotiate a truce. Aliyev set uncompromising conditions Sunday for a halt to the fighting, saying Armenian forces “must leave our territories, not in words but in deeds, apologize to the Azerbaijani people and recognize the territorial integrity of Azerbaijan".