At least 16 military personnel and several civilians were killed in intense fighting between neighbors Armenia and Azerbaijan over a disputed region.
They were the worst clashes since 2016 and re-sparked stability concerns in the South Caucasus, a corridor for pipelines that transport oil and gas to world markets.
The clashes between the two former Soviet republics that waged war in the 1990s were the latest flare-up in a long-running conflict over Nagorno-Karabakh, a breakaway region within Azerbaijan, but led by ethnic Armenians.
Nagorno-Karabakh said 16 of its soldiers were killed and more than 100 injured after Azerbaijan launched an air and artillery attack early Sunday.
Armenia and Nagorno-Karabakh declared martial law and mobilized the male population.
Azerbaijan, which also declared martial law, said its armed forces responded to the shelling by Armenians and that five members of a family were killed by shelling by Armenians.
It also said that its armed forces had taken control of up to seven villages. Nagorno-Karabakh initially denied this but later admitted it had lost “some positions” and said it had suffered a number of civilian casualties without giving details.
The Azerbaijani public prosecutor said five members of a family had been killed by shelling by Armenians.
The clashes sparked a flurry of diplomacy to ease the new tensions in a decade-long conflict between majority Christians
Armenia and mostly Muslim Azerbaijan, with Russia calling for an immediate ceasefire and another regional power, Turkey, to support Azerbaijan.
The US condemned the escalation and called on both sides to end hostilities immediately.
Pipelines transporting Caspian oil and natural gas from Azerbaijan to the world run near Nagorno-Karabakh.
Armenia also warned of security risks in the South Caucasus in July after Azerbaijan threatened to attack the Armenian nuclear power plant as possible retaliation.
Nagorno-Karabakh broke away from Azerbaijan in a conflict that broke out when the Soviet Union collapsed in 1991.
Although a ceasefire was agreed in 1994, Azerbaijan and Armenia frequently accuse each other of attacks around Nagorno-Karabakh and along the separate Azerbaijani-Armenian border after the deaths of thousands of people and many more displaced persons.
During the clashes on Sunday, Armenian right-wing activists said an ethnic Armenian woman and child were also killed.
Armenia said the Azerbaijani armed forces had attacked civilian targets like the capital of Nagorno-Karabakh, Stepanakert, and promised an “appropriate response”.
“We remain strong alongside our army to protect our motherland from the invasion of Azerbaijan,” wrote Armenian Prime Minister Nikol Pashinyan on Twitter.
Azerbaijan denied a statement by the Armenian Defense Ministry that Azerbaijani helicopters and tanks had been destroyed, and accused the Armenian armed forces of “deliberate and targeted” attacks along the front line.
“We are defending our territory, our cause is right,” said Azerbaijani President Ilham Aliyev in an address to the nation.
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Turkey said it spoke with members of the Minsk group, which mediates between Armenia and Azerbaijan. Russia, France and the United States are co-presidents.
Russian President Vladimir Putin spoke to Pashinyan by phone, but details of the conversation were not known, and Turkish President Tayyip Erdogan spoke to Aliyev.
Erdogan, who pledged support for traditional ally Azerbaijan, said Armenia was “the greatest threat to peace in the region” and urged “the whole world to stand with Azerbaijan in the fight against invasion and cruelty”.
Pashinyan struck back and called on the international community to ensure that Turkey does not become involved in the conflict.
The European Union and the Organization for Security and Co-operation in Europe (OSCE) called on both sides to cease military action and return to the negotiations, as did Pope Francis.
At least 200 people were killed when the conflict between Armenia and Azerbaijan flared up in April 2016. At least 16 people were killed in clashes in July.