Soldiers in the Azerbaijani army man an outpost overlooking the Armenian border near Kedabek, Azerbaijan on Sept. 1, 1992.

Arch foes Armenia and Azerbaijan said Saturday fierce clashes had erupted along the front line of Nagorny Karabakh, with Russia urging an immediate ceasefire to end the latest surge of violence over the disputed region.

Armenia’s defense ministry said that “Azerbaijan on Friday night launched a massive attack along the Karabakh frontline using tanks, artillery, and helicopters.” Armenian-backed separatist forces meanwhile claimed to have shot down two Azeri helicopters.

“As a result of the Karabakh army’s counterstrike, Azeri forces were repelled and sustained serious losses in manpower,” a defense ministry statement said. “Heavy fighting is underway.”

Azerbaijan denied an earlier claim that one of its helicopters was brought down.

It insisted its forces were counter-attacking after coming under fire from “large-calibre artillery and grenade-launchers” by the Armenian side.

“Azerbaijani armed forces have immediately undertaken retaliatory measures against Armenian forces,” Azerbaijan’s defense ministry said in a statement, adding that “fighting continues in the directions of Agdere-Terter-Agdam and Khojavend-Fizuli.”

The Armenian-backed defense 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.

The two sides frequently exchange fire across the frontline but the latest episode appeared to be an intensification from the usual sporadic clashes.

In a sign that the situation was spiraling Russian President Vladimir Putin called for an immediate end to fighting along the front line, 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.

Armenian Prime Minister Ovik Abrahamyan convened an urgent meeting over the “unprecedented large-scale hostilities from the enemy”, the Armenian government said.

“Armenia is ready to take the necessary steps to stabilize the situation,” Abraamyan said.

Ethnic Armenian separatists backed by Yerevan seized control of the mountainous Nagorny Karabakh region, which was part of Azerbaijan during the Soviet period, during a war in the early 1990s which claimed the lives of some 30,000 people.

A 1994 ceasefire failed to lead to a peace deal, with clashes erupting regularly and the two countries remaining on a war footing.

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

Energy-rich Azerbaijan, whose military spending has 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.

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