Yerevan (AFP) - Fierce clashes left at least 30 Azerbaijani and Armenian soldiers dead on Saturday, prompting Russia and the West to call for an immediate ceasefire after a bloody escalation over the disputed Nagorny Karabakh region.

Armenian President Serzh Sarkisian said 18 Armenian troops were killed and some 35 wounded in the "largest-scale hostilities" since a 1994 truce ended a war in which Armenian-backed fighters seized the territory from Azerbaijan.

Sarkisian did not specify if the troops were from the forces of unrecognised Karabakh -- which claims independence but is backed by Yerevan -- or Armenia's army.

Earlier Azerbaijan's defence ministry said that 12 of its soldiers were killed in the clashes and a military helicopter shot down.

The surge in fighting over the disputed territory reportedly also claimed the lives of one Armenian and one Azeri civilian after the arch foes accused each other of unleashing heavy weaponry over the volatile frontline.

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".

Azerbaijan defence ministry spokesman Vagif Dargahli said that firing had stopped by Saturday evening but warned that the situation remained highly volatile.

Russian President Vladimir Putin called for an immediate end to fighting along the frontline.

"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.

Moscow has supplied weaponry to both sides in the conflict, but has much closer military and economic ties to Armenia and Yerevan is reliant on Russia's backing.

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.

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- 'No military solution' -

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 negotiate a settlement, also expressed "grave concern".

The European Union's foreign policy chief Federica Mogherini said 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".

US Secretary of State John Kerry called on both sides to return to peace talks under the auspices of the OSCE, reiterating "there is no military solution to the conflict".

Azerbaijan's strongman President Ilham Aliyev also spoke by phone to ally Turkish President Recep Tayyip Erdogan, who expressed "solidarity" with Azerbaijan, Aliyev's press office said.

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 Aliyev and 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 emphasised the importance of a comprehensive settlement for the long-term stability, security, and prosperity of the region", the White House said.

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

The region is still internationally recognised as part of Azerbaijan and the two sides frequently exchange fire, but the latest episode marked a surge in violence and sparked frantic appeals for peace from international powers.

Azeri forces claimed that they had taken control of several strategic heights and a village in the Armenian-controlled territory, but Yerevan denied the claim as "disinformation".