At least 24 people have died after deadly clashes between arch foes Armenia and Azerbaijan, as the latest violence in the decades-long territorial dispute sparked international calls Sunday to halt the fighting.

The worst skirmishes since 2016 have raised the spectre of a fresh war between the ex-Soviet rivals, locked since the early 1990s in a stalemate over the Armenia-backed breakaway region of Nagorny Karabakh.

Seventeen Armenian separatist fighters were killed and more than 100 wounded in the fighting, Karabakh President Arayik 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, adding that he was waiting to be conscripted into the army.

Karabakh separatists said one Armenian woman and a child were killed, while Baku said that an Azerbaijani family of five died in shelling launched 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 "some 200 Azerbaijani troops and destroyed 30 enemy artillery units and 20 drones".

- 'Full-scale war' -

Fighting between Muslim Azerbaijan and majority-Christian 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," Pashinyan warned.

France, Germany, Italy, and the European Union 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 flareup with Pashinyan and called for "an end to hostilities".

But Azerbaijan's ally Turkey blamed Yerevan for the flare-up 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.

Karabakh President Harutyunyan said Turkey was providing mercenaries and warplanes to the fight, suggesting, "the war has already... (gone) beyond the limits of a Karabakh-Azerbaijan conflict."

Azerbaijan accused Armenian forces of violating a ceasefire, saying it had launched a counter-offensive to "ensure the safety of the population", using tanks, artillery missiles, combat aviation and drones.

- Curfew, martial law -

In a televised address to the nation earlier Sunday, Azerbaijan President Ilham Aliyev vowed victory over Armenian forces.

"Our cause is just and we will win," he said, echoing a famous quote from Soviet dictator Joseph Stalin's address at the outbreak of World War II in Russia.

"Karabakh is Azerbaijan."

Both Armenia and Karabakh declared martial law and military mobilisation. Azerbaijan imposed military rule and a curfew in large cities.

Armenia said that Azerbaijan attacked civilian settlements in Nagorny 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 Nagorny Karabakh region from Baku in a 1990s war that claimed 30,000 lives.

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 United States have mediated peace efforts as the "Minsk Group" but the last big push for a peace deal collapsed in 2010.

- Frontline bombing -

"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 along Karabakh's frontline including civilian targets and in Stepanakert, Karabakh's presidency said.

The rebel defence ministry said its troops shot down four Azerbaijani helicopters and 15 drones, while Baku denied the claim.

In July, heavy clashes along the two countries' shared border -- hundreds of kilometres from Karabakh -- claimed the lives of at least 17 soldiers from both sides.

During the worst recent Karabakh clashes in April 2016, around 110 people were killed.