Decades-old tensions erupt again into heavy clashes, prompting fears of an all-out conflict.
The contested Nagorno-Karabakh region, a mountainous and heavily-forested patch of land, is at the heart of a decades-long armed standoff between neighbours Armenia and Azerbaijan.
Under international law, Nagorno-Karabakh is recognised as part of Azerbaijan. But the ethnic Armenians who make up the vast majority of the population reject Azerbaijani rule. They have been running their own affairs, with support from Armenia, since Azerbaijan’s forces were pushed out in a war in the 1990s.
On Sunday, heavy clashes in Nagorno-Karabakh prompted fears that the dispute could spiral once again into all-out war.
The status of the region has been disputed at least since 1918, when Armenia and Azerbaijan became independent from the Russian empire.
In the early 1920s, Soviet rule was imposed in the south Caucasus and the predominantly Armenian-populated Nagorno-Karabakh became an autonomous region within the then-Soviet republic of Azerbaijan, with most decisions being made in Moscow.
But decades later, as the Soviet Union started to crumble, it became apparent that Nagorno-Karabakh would come under the direct rule of the government in the Azerbaijani capital, Baku. The ethnic Armenians did not accept that.
In 1988, the Nagorno-Karabakh legislature voted to join the Armenian republic, a demand strongly opposed by both the Azerbaijani Soviet government and Moscow.
After the collapse of the Soviet Union in 1991, Yerevan-backed Armenian separatists seized the territory, home to a significant Azerbaijani minority, as well as seven adjacent Azerbaijani districts.
At least 30,000 people were killed and hundreds of thousands were forced from their homes in the fighting.
Despite an internationally-brokered ceasefire agreed in 1994, peace negotiations have stalled and clashes erupt frequently around Nagorno-Karabakh and along the Azerbaijan-Armenia border.
In April 2016, dozens of people from both sides were killed in the most serious fighting in Nagorno-Karabakh in years.
The latest clashes on Sunday also left both sides with casualties, including civilians.
They follow a flare-up along the Azerbaijan-Armenia border in July, which killed at least 17 soldiers from both sides.
The long-running conflict has concerned the international community in part because of its threat to stability in a region that serves as a corridor for pipelines taking oil and gas to world markets.