Azerbaijan and Armenian forces engaged in fierce clashes Sunday when a decadeslong conflict over disputed land erupted into renewed war involving tanks, artillery and aircraft.
Russia and international organizations including NATO, the European Union and the OSCE called on both sides to halt fighting over the territory of Nagorno-Karabakh. Turkey backed its ally Azerbaijan and said it was ready to offer assistance.
Armenian Prime Minister Nikol Pashinyan declared martial law and ordered a general mobilization, after accusing Azerbaijan of "preplanned aggression." Azeri President Ilham Aliyev, who also announced martial law, said in a state TV address that Armenian forces were occupying Azerbaijan's territory and "we'll put an end to this occupation."
Conflict has broken out repeatedly since Armenians took control of Nagorno-Karabakh and seven surrounding districts from Azerbaijan in a war after the Soviet Union collapsed in 1991. Hundreds died in 2016 before Russia pressured both sides to resume a cease-fire it first brokered in 1994. Armenia and Azerbaijan clashed again across their state border in July.
The confrontation has the potential to drag in Russia and Turkey, adding to geostrategic tensions between them over proxy conflicts in Syria and Libya. Russia has a mutual-defense pact with Armenia and a military base in the republic, while Azerbaijan hosted large-scale joint military exercises with Turkish forces last month.
Russian President Vladimir Putin expressed "serious concern about the renewal of large-scale military conflict" and called for a halt to hostilities in a phone call with Pashinyan, according to a Kremlin statement.
"Turkey stands by its Azeri brothers with all its means," Turkish President Recep Tayyip Erdogan said on Twitter after he spoke with Aliyev.
Russian Foreign Minister Sergei Lavrov discussed the fighting with his Turkish counterpart Mevlut Cavusoglu, the Foreign Ministry in Moscow said in a statement. Lavrov also spoke separately by phone with the Armenian and Azeri foreign ministers, emphasizing the need for an immediate cease-fire, the ministry said.
A BP Plc-operated oil pipeline runs less than 30 miles (50 kilometers) from the conflict zone and carries as much as 1.2 million barrels daily from Baku to Turkey's Ceyhan. While it hasn't been targeted in previous conflicts, the pipeline may be vulnerable to any shift in the fighting between Armenian and Azeri forces.
Despite decades of mediation by the U.S., France and Russia, the two sides have never signed a peace agreement. Armenia says the right of the internationally unrecognized Nagorno-Karabakh Republic to self-determination should be respected, while Azerbaijan says its territorial integrity must be upheld.