WHAT HAS HAPPENED ?
Fighting has broken out between Armenia and Azerbaijan, republics in the Caucasus that have gone to war twice over the disputed region of Nagorno-Karabakh.
In a sharp escalation of hostilities, Armenia said nearly 50 of its soldiers had been killed in clashes along the border with Azerbaijan.
Both sides blame each other for the most deadly flare-up since the 2020 war, and world powers have urged a ceasefire.
WHAT'S THE DISPUTE IN THE REGION ?
Nagorno-Karabakh is a mountainous, landlocked region inside the borders of Azerbaijan that has been a source of dispute for more than a century.
The area is recognised internationally as Azerbaijan's territory but has a primarily Armenian population who have resisted Azerbaijani rule.
In 1991, the region of about 150,000 people declared independence and since then it has ruled itself - with Armenian support - as the unrecognised Republic of Artsakh. Despite signs in recent years of progress towards peace, the "frozen conflict" erupted again in 2020.
Azerbaijan reclaimed broad swaths of Nagorno-Karabakh in a six-week war that killed more than 6,600 people and ended with a Russia-brokered peace deal. Moscow deployed about 2,000 troops to the region to serve as peacekeepers.
Azerbaijan is majority Muslim and Armenia is majority Christian, and some elements on both sides seek to cast the conflict in religious terms, though analysts say this angle is exaggerated.
WHY ARE THEY FIGHTING NOW ?
An Armenian revolution in 2018 ushered in a new generation of leadership and raised hopes that the Nagorno-Karabakh conflict could move towards a resolution.
Those aspirations have since dwindled, with Armenia's prime minister, Nikol Pashinyan, taking a firm - and in the eyes of Azerbaijani leaders, provocative - line on the issue.
Azerbaijan, which has been under the grip of a single family since 1993, says it is responding to Armenian aggression in areas that are legally its territory and which have been occupied by enemy troops and separatists for decades.
WHAT HAPPENED IN THE RECENT CONFLICT ?
Armenia said that Azerbaijani forces "launched intensive shelling, with artillery and large-calibre firearms, against Armenian military positions in the direction of the cities of Goris, Sotk, and Jermuk" shortly after midnight.
Azerbaijan's defence ministry accused Armenia of "large-scale subversive acts" near the districts of Dashkesan, Kelbajar and Lachin on the border, adding that its army positions "came under fire, including from trench mortars".
How did world leaders react ?
⚫ Turkey, a close ally of Azerbaijan, told Yerevan to "cease its provocations" against Baku, with foreign minister Mevlut Cavusoglu saying on Twitter that the country should instead "focus on peace negotiations and cooperation" with its neighbour.
⚫ US Secretary of State Antony Blinken said Washington was deeply concerned about reports of the attacks, and urged "an end to any military hostilities immediately", adding that "there can be no military solution to the conflict". Russia said it was "extremely concerned" by the uptick in fighting.
⚫ Charles Michel, the president of the European Council, said the bloc was "ready to make efforts to prevent further escalation", adding that there was "no alternative to peace and stability in the region".
WHY DOES THE CONFLICT MATTER ?
Other than the humanitarian issue, with civilians on both sides being killed, the conflict sparks international concern for a few reasons.
The wider south Caucasus is a crucial artery for gas and oil from Azerbaijan into Turkey and on to Europe and other world markets. Regional powers including Russia, Turkey and Iran are invested in the south Caucasus to varying degrees.
Turkey has already declared its staunch support for Turkic-speaking Azerbaijan, while Russia has a security alliance with Armenia, though it sells weapons to both countries.
Moscow and Ankara have been jostling for influence in different theatres around the world including in Syria and Libya. Furthermore, Moscow's stake in the Nagorno-Karabakh conflict - while also fighting in Ukraine - adds a fresh element of uncertainty to the crisis.
