The worst clashes since 2016 have raised the specter of a new war between Azerbaijan and Armenia, which have been locked for decades in a territorial dispute over the Armenia-backed breakaway region of Nagorno-Karabakh.
A major confrontation between Muslim Azerbaijan and Christian Armenia threatened to embroil regional players Russia and Turkey, according to Al Jazeera.
Amid the rising tensions, the United States, Russia, Turkey, France and others have reacted to Sunday's fighting over the disputed region of Nagorno-Karabakh.
The US State Department said it contacted the two countries "to urge both sides to cease hostilities immediately, to use the existing direct communication links between them to avoid further escalation, and to avoid unhelpful rhetoric and actions".
France, a co-chair of the OSCE Minsk Group mediating between Armenia and Azerbaijan in a bid to find a peaceful solution to the decades-old Nagorno-Karabakh conflict, called on Yerevan and Baku to end hostilities and immediately restart dialogue.
Fellow Minsk Group co-chair Russia also called for an immediate ceasefire. "We are calling on the sides to immediately halt fire and begin talks to stabilize the situation," the Russian foreign ministry said.
Russian President Vladimir Putin discussed the military flareup with Armenian Prime Minister Nikol Pashinyan and called for "an end to hostilities". Foreign Minister Sergey Lavrov, meanwhile, was in intensive talks with his Turkish counterpart to reach a return to negotiations.
Azerbaijan's ally Turkey has blamed Armenia for the flare-up and promised Azerbaijan its "full support".
"While I call on the Armenian people to take hold of their future against their leadership that is dragging them to catastrophe and those using it like puppets, we also call on the entire world to stand with Azerbaijan in their battle against invasion and cruelty," Turkish President Recep Tayyip Erdogan said on Twitter, adding Turkey will "increasingly continue" its solidarity with Baku.
Earlier, his spokesman Ibrahim Kalin accused Armenia of violating a ceasefire with Azerbaijan "by attacking civilian locations".
At the same time, Turkish Defense Minister Hulusi Akar said recent hostilities could end "throw[ing] the region into fire" while promising Ankara's backing to Baku. "The biggest obstacle to peace and stability in the Caucasus is the hostile stance of Armenia, and it must immediately turn back from this hostility that will send the region into fire," Akar said. "We will support our Azerbaijani brothers with all our means in their fight to protect their territorial integrity," he added.
Relations between the two former Soviet republics have been tense since 1991 when Armenian military occupied the Upper Karabakh, or Nagorno-Karabakh region, an internationally recognized territory of Azerbaijan.
Republic of Artsakh
With the support of Armenia, Nagorno-Karabakh enclave was declared in 1991 as the Republic of Artsakh, which has a population of around 145,000 people. Although internationally recognized as Azeri territory, Baku has had no control over the region since 1988.
The predominantly Armenian-populated region of Nagorno-Karabakh was claimed by both the Azerbaijan Democratic Republic and the First Republic of Armenia when both countries became independent in 1918 after the fall of the Russian Empire, and a brief war over the region broke out in 1920. The dispute was largely shelved after the Soviet Union established control over the area, and created the Nagorno-Karabakh Autonomous Oblast (NKAO) within the Azerbaijan SSR in 1923. During the fall of the Soviet Union, the region re-emerged as a source of dispute between Armenia and Azerbaijan. In 1991, a referendum held in the NKAO and the neighboring Shahumian region resulted in a declaration of independence. Ethnic conflict led to the 1991-1994 Nagorno-Karabakh War, which ended with a ceasefire along roughly the current borders.
In his 2015 speech, the President of Armenia Serzh Sargsyan stated that he considered Nagorno-Karabakh "an inseparable part of Armenia".
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