Azerbaijan launched "anti-terrorist activities" in the Nagorno-Karabakh region on Tuesday, saying it wanted to restore constitutional order and drive out what it said were Armenian troops, a move that could foreshadow a new war.
Armenia and Azerbaijan have already fought two wars over Karabakh in the three decades since the Soviet Union they were both members of collapsed.
Here is a look at the history of the conflict and the latest developments.
WHAT IS NAGORNO-KARABAKH?
Nagorno-Karabakh, known as Artsakh by Armenians, is a mountainous region at the southern end of the Karabakh mountain range, within Azerbaijan. It is internationally recognised as part of Azerbaijan, but its 120,000 inhabitants are predominantly ethnic Armenians. They have their own government which is close to Armenia but not officially recognised by Armenia or any other country.
Armenians, who are Christian, claim a long presence in the area, dating back to several centuries before Christ. Azerbaijan, whose inhabitants are mostly Turkic Muslims, also claims deep historical ties to the region, which over the centuries has come under the sway of Persians, Turks and Russians. Bloody conflict between the two peoples goes back more than a century.
Under the Soviet Union, Nagorno-Karabakh became an autonomous region within the republic of Azerbaijan.
FIRST KARABAKH WAR
As the Soviet Union crumbled, the First Karabakh War (1988-1994) erupted between Armenians and their Azeri neighbours. About 30,000 people were killed and more than a million displaced. Most of those were Azeris driven from their homes when the Armenian side ended up in control of Nagorno-Karabakh itself and swathes of seven surrounding districts.
44-DAY WAR IN 2020
In 2020, after decades of intermittent skirmishes, Azerbaijan began a military operation that became the Second Karabakh War, swiftly breaking through Armenian defences. It won a resounding victory in 44 days, taking back the seven districts and about a third of Nagorno-Karabakh itself.
The use of drones bought from Turkey and Israel was cited by military analysts as one of the main reasons for Azerbaijan's victory. At least 6,500 people were killed.
Russia, which has a defence treaty with Armenia but also has good relations with Azerbaijan, negotiated a ceasefire.
The deal provided for 1,960 Russian peacekeepers to guard the territory's lifeline to Armenia: the road through the "Lachin corridor", which Armenian forces no longer controlled.
Analysts say successive rounds of talks, mediated variously by the European Union, the United States and Russia, have brought the two sides closer to a permanent peace treaty than they have been for years, but a final settlement remains elusive. The most sensitive issue is the status of the 120,000 ethnic Armenians in Karabakh, whose rights and security Armenia says must be guaranteed. Prime Minister Nikol Pashinyan has said Armenia recognises the sovereignty and territorial integrity of Azerbaijan, but Baku says it is not certain the assertion was made in good faith and accuses Armenia of fuelling separatism.
In December 2022 Azerbaijani civilians identifying themselves as environmental activists began blocking the Lachin corridor, and in April 2023 Azerbaijan set up an official checkpoint, saying it was preventing weapons smuggling. The flow of people and goods between Armenia and Nagorno-Karabakh was largely cut off. The United States bemoaned the "rapidly deteriorating humanitarian situation".
This week, the International Committee of the Red Cross (ICRC) was able to make simultaneous aid deliveries via the Lachin corridor and a separate road linking Karabakh to the Azerbaijani city of Aghdam.
Despite that, tensions have risen sharply this month, with Armenia and Azerbaijan accusing each other of building up troops.
Armenia has complained loudly that Russia's war in Ukraine has distracted it from what Moscow itself says is its role as the guarantor of security in the South Caucasus.