At least 26 more fighters have been killed in clashes with Azerbaijani troops in the breakaway region of Nagorno-Karabakh, according to its defence ministry, bringing their military death toll to 84 in the fiercest round of fighting in more than a quarter of a century.

According to reports, the total number of civilians killed has risen to 11 – nine in Azerbaijan and two in Armenia, bringing the total death toll to 95.

Fierce fighting raged between Azerbaijani and Armenian forces, sparking bellicose rhetoric from regional power Turkey despite international pleas for a halt in fighting between the long-time enemies.

Yerevan and Baku have been locked in a territorial dispute over the ethnic Armenian region of Nagorny Karabakh for decades, with deadly fighting flaring up last July and in 2016.

The region declared independence from Azerbaijan after a war in the early 1990s that claimed 30,000 lives but it is not recognised by any country -- including Armenia -- and is still considered part of Azerbaijan by the international community.

On Monday evening, Azerbaijani forces launched a "massive offensive at the Karabakh frontline's southern and north-eastern sectors," said Armenia's defence ministry spokesman Artsrun Hovhannisyan.

Karabakh's defence ministry said 26 more of its troops were killed late on Monday, bringing the rebels' total military losses to 84. 

The overall death toll rose to 95, including 11 civilian deaths: nine in Azerbaijan and two on the Armenian side.

Azerbaijan has not reported any military casualties, but Armenian separatist officials released footage showing burnt-out armoured vehicles and the bloodied and charred remains of soldiers in camouflage it said were Azerbaijani troops

Fighting between majority-Muslim Azerbaijan and Christian Armenia could embroil regional players, Russia and Turkey.

Russia, which has a military alliance with Armenia and stations there a permanent military base, sells sophisticated weapons worth billions of dollars to both Baku and Yerevan.

Armenia has accused Turkey -- which backs Turkic-speaking Azerbaijan -- of meddling in the conflict.

Azerbaijani President Ilham Aliyev on Monday ordered partial military mobilisation and General Mais Barkhudarov vowed to "fight to the last drop of blood in order to completely destroy the enemy and win". 

With each side blaming the other for the flare-up, world leaders have urged calm as fears rise of a full-scale conflict.

The UN Security Council was to meet on Tuesday at 5:00 pm (2100 GMT) for emergency talks on Karabakh behind closed doors, diplomats told AFP.

Kremlin spokesperson Dmitry Peskov said Russia was monitoring the situation closely and that the current priority was to "stop the hostilities, not to deal with who is right and who is wrong".

But Turkish President Recep Tayyip Erdogan demanded Armenia end its "occupation" of Karabakh.

"The time has come for the crisis in the region that started with the occupation of Nagorny Karabakh to be put to an end," Erdogan said. 

"Now Azerbaijan must take matters into its own hands."

- Mercenaries from Syria -
Armenia has accused Turkey of sending mercenaries to back Azerbaijan.

A war monitor Monday said Turkey has sent at least 300 proxies from northern Syria to join Azerbaijani forces.

Turkey informed the fighters they would be tasked with "guarding border regions" in Azerbaijan in return for wages of up to $2,000, said Rami Abdul Rahman, the head of the Britain-based Syrian Observatory for Human Rights.

The report comes after the European Union warned regional powers not to interfere in the fighting and condemned a "serious escalation" that threatens regional stability.

In addition to the EU and Russia, France, Germany, Italy and the United States have urged a ceasefire.

Armenian defence ministry spokeswoman Shushan Stepanyan said Armenian separatist forces had won back positions taken Sunday by Azerbaijan.

But Baku claimed further advances.

Azerbaijani forces "are striking enemy positions... and have taken several strategic positions around the village of Talysh", the defence ministry said.

"The enemy is retreating," it added, accusing separatist forces of shelling civilian targets in the town of Terter.

- 'We are not afraid of war' -
The escalation has stirred an outpouring of patriotic fervour in both countries.

"We have been waiting for this day for so long. The fighting must not stop until we force Armenia to return our lands," Vidadi Alekperov, a 39-year-old waiter in Baku, told AFP.

"I'll happily go to the battlefield." 

In Yerevan, 67-year-old Vardan Harutyunyan said Armenia had been anticipating the attack.

"The (Karabakh) question can only be resolved militarily. We are not afraid of a war," he said.

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

Talks to resolve the conflict -- one of the worst to emerge from the 1991 collapse of the Soviet Union -- have largely stalled since a 1994 ceasefire agreement.

Analysts told AFP that international brokers needed to step up efforts to prevent an even worse escalation.

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.