Turkey sent military experts, drones and warplanes to reinforced the Azerbaijan in fighting with neighbouring Armenia over disputed territory, the Armenian Foreign Ministry said on Monday.
The Armenian parliament accused Turkey of interfering in the conflict, which Azerbaijan denied.
Fighting between Armenia and Azerbaijan over the breakaway region of Nagorno-Karabakh has killed dozens and raged into Monday morning with heavy artillery used by both sides.
The Armenian defence ministry reported fighting throughout the night, while its counterpart in Azerbaijan said Armenian forces were shelling the town of Terter.
Karabakh President Arayik Harutyunyan said Turkey was providing mercenaries and warplanes.
“The war has already ... [gone] beyond the limits of a Karabakh-Azerbaijan conflict,” he said.
The skirmishes have raised the spectre of a new war between the ex-Soviet rivals, locked since the early 1990s in a stalemate over the Armenia-backed breakaway region of Nagorno-Karabakh.
Seventeen Armenian separatist fighters were killed and more than 100 wounded in the fighting, Mr Harutyunyan said, conceding that his forces had lost positions.
Both sides also reported civilian casualties.
“We are tired of Azerbaijan’s threats, we will fight to the death to resolve the problem once and for all,” Artak Bagdasaryan, 36, told AFP in Yerevan.
He was waiting to be conscripted into the army, he said.
Karabakh separatists said one Armenian woman and a child were killed, while Baku said an Azerbaijani family of five died in shelling by Armenian separatists.
Azerbaijan claimed it captured a strategic mountain in Karabakh that helps control transport links between Yerevan and the enclave.
Armenian defence ministry spokesman Artsrun Hovhannisyan, in turn, said Karabakh rebel forces killed about 200 Azerbaijani troops and destroyed 30 enemy artillery units and 20 drones.
Fighting between Muslim Azerbaijan and Christian-majority Armenia threatened to embroil regional players Russia and Turkey, with Armenian Prime Minister Nikol Pashinyan calling on global powers to prevent Ankara’s involvement.
“We are on the brink of a full-scale war in the South Caucasus,” Mr Pashinyan said.
France, Germany, Italy and the EU swiftly urged an immediate ceasefire, while Pope Francis prayed for peace.
French President Emmanuel Macron expressed his deep concern on Sunday and “strongly called for an immediate end to hostilities”.
UN Secretary General Antonio Guterres said he was also extremely concerned and urged the sides to stop fighting and return to talks.
The US State Department said it had contacted the two countries and called on them to “use the existing direct communication links between them to avoid further escalation”.
Russian President Vladimir Putin discussed the military flare with Mr Pashinyan and called for an end to hostilities.
But Azerbaijan’s ally Turkey blamed Yerevan for the fighting and promised Baku its full support.
“The Turkish people will support our Azerbaijani brothers with all our means as always,” Turkish President Recep Tayyip Erdogan tweeted.
Azerbaijan accused Armenian forces of breaching a ceasefire, saying it had launched a counteroffensive to “ensure the safety of the population”, using tanks, artillery missiles and drones.
In a televised address to the nation earlier on Sunday, Azerbaijani President Ilham Aliyev vowed victory over Armenian forces.
“Our cause is just and we will win,” he said, echoing Soviet dictator Joseph Stalin’s address at the outbreak of the Second World War.
“Karabakh is Azerbaijan.”
Armenia and Karabakh declared martial law and military mobilisation. Azerbaijan imposed military rule and a curfew in cities.
Armenia said that Azerbaijan attacked civilian settlements in Nagorno-Karabakh including the main city, Stepanakert.
Azerbaijan’s foreign ministry said there were reports of dead and wounded. “Extensive damage has been inflicted on many homes and civilian infrastructure,” it said.
Ethnic Armenian separatists seized the Nagorno-Karabakh region from Baku in the 1990s, a war in which 30,000 were killed.
Talks to resolve one of the worst conflicts to emerge from the 1991 collapse of the Soviet Union have been largely stalled since a 1994 ceasefire agreement.
France, Russia and the US have mediated peace efforts as the Minsk Group, but the last big push for a peace deal collapsed in 2010.
“We are a step away from a large-scale war,” Olesya Vartanyan of the International Crisis Group told AFP.
“One of the main reasons for the current escalation is a lack of any proactive international mediation ... for weeks.”
On Sunday morning, Azerbaijan started bombing Karabakh’s front line, including civilian targets, and Stepanakert, Karabakh’s presidency said.
The rebel defence ministry said its troops shot down four Azerbaijani helicopters and 15 drones, which Baku denied.
In July, heavy clashes along the countries’ shared border – hundreds of kilometres from Karabakh – killed an Azerbaijani civilian and at least 16 soldiers in total, with losses on both sides.
During clashes in April 2016, about 110 people were killed.