Yerevan (AFP) - Azerbaijan said Saturday at least 12 of its soldiers were killed and a helicopter downed by Armenian forces, in a major escalation of violence over the Nagorny Karabakh region, prompting Russia and the West to urge an immediate ceasefire.

The surge in fighting over the disputed territory -- which separatist Armenian-backed forces captured from Azerbaijan in the early 1990s -- reportedly also claimed the lives of one Armenian and one Azeri civilian after the arch foes accused each other of unleashing heavy weaponry across the volatile front line.

Armenia accused Azerbaijan of launching a "massive attack along the Karabakh frontline using tanks, artillery, and helicopters" on Friday night.

Azerbaijan, however, insisted it had counter-attacked after coming under fire from "large-calibre artillery and grenade-launchers".

Ethnic Armenian separatists backed by Yerevan seized control of the mountainous Nagorny Karabakh region in an early 1990s war that claimed some 30,000 lives and the two sides have never signed a peace deal despite a 1994 ceasefire.

They frequently exchange fire across the front but the latest episode marked a surge in violence and sparked frantic appeals for peace from international powers.

Initial claims from the Armenian capital Yerevan were that the fighting involved Azerbaijani forces and Armenian-backed separatist forces, but Azerbaijan said it was battling "Armenian forces".

"Twelve Azeri servicemen were killed in action and a helicopter was shot down by Armenian forces," Azerbaijan's defence ministry said in a statement, adding that a tank had also been blown up on a mine.

The defence ministry claimed Azeri forces "took control of the strategic Lala-Tepe height" as well as "heights near the village of Talysh as well as Seysulan village" inside the Armenian-controlled territory.

Armenia, however, denied that Azeri forces had gained ground and rubbished claims from Baku that 100 of its troops had been killed in the clashes.

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The Armenian-backed defence ministry in Karabakh said a 12-year-old boy was killed and two other children injured on their side, while Azerbaijan's foreign ministry said one Azeri civilian died.

- Ceasefire calls -

In a sign that the situation was spiralling, Russian President Vladimir Putin called for an immediate end to fighting along the frontline, the Kremlin said.

"President Putin calls on the parties in the conflict to observe an immediate ceasefire and exercise restraint in order to prevent further casualties," Kremlin spokesman Dmitry Peskov told Russian news agencies.

Foreign Minister Sergei Lavrov and Defence Minister Sergei Shoigu held phone talks with their counterparts in Armenia and Azerbaijan to urge a de-escalation in the fighting.

Meanwhile, mediators from a group made up of representatives from Russia, the United States, France and the Organisation for Security and Cooperation in Europe (OSCE), which has been trying to negotiation a settlement, expressed "grave concern".

The European Union's foreign policy chief Federica Mogherini said that the reports of heavy fighting were "deeply worrying" and called on all sides to "avoid any further actions or statements that could result in escalation".

Energy-rich Azerbaijan, whose military spending has in the past exceeded Armenia's entire state budget, has repeatedly threatened to take back the breakaway region by force if negotiations fail to yield results. Moscow-backed Armenia says it could crush any offensive.

The last big flare-up occurred in November 2014 when Azerbaijan shot down an Armenian military helicopter.

US Vice President Joe Biden met this week separately with both Azerbaijan's President Ilham Aliyev and the leader of Armenia, Serzh Sarkisian, as they attended a nuclear summit in Washington. He urged a peaceful settlement to the dispute.

Biden "expressed concern about continued violence, called for dialogue, and emphasized the importance of a comprehensive settlement for the long-term stability, security, and prosperity of the region", the White House said.