Men are seen in the yard of a damaged house in the village of Djojugh Marjanli in Nagorny Karabakh region. AFP

At least 28 pro-Turkey Syrian rebel fighters have been killed in clashes between Armenian and Azerbaijani forces over Nagorny Karabakh, a Britain-based monitor said on Friday.

They were among more than 850 combatants from pro-Ankara Syrian factions that Turkey has sent to fight for the Azeris since last week, the Syrian Observatory for Human Rights said.

Relatives of three fighters confirmed to AFP they had been killed, while social media users shared pictures of four fighters who had died in the clashes.

Armenia has accused Turkey of dispatching Syrian fighters to fight on the Azeri side in Nagorny Karabakh, despite Azerbaijani and Turkish denials.

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French President Emmanuel Macron demanded Friday that Turkey explain what he said was the arrival of militant fighters in Azerbaijan.

Intelligence reports had established that 300 Syrian fighters drawn from "militant groups" from the Syrian city of Aleppo had passed through Gaziantep in Turkey en route for Azerbaijan, he said.

Servicemen of Armenia stand outside a damaged ministry's building in Nagorny Karabakh region. AFP

Clashes have raged between Armenian and Azerbaijani forces since Sunday over Nagorny Karabakh, an ethnic Armenian province that broke away from Baku in a bitterly fought war in the 1990s.

The rival Caucasus nations have been locked in a bitter stalemate over Karabakh since the collapse of the Soviet Union when the ethnic Armenian region broke away from Azerbaijan.

The fiercest clashes between Armenian and Azerbaijani forces in years ignited Sunday and confirmed deaths reached 136 as fighting raged for a fifth day.

The Armenian defence ministry said fighting had "intensified" and its troops had repelled Azerbaijani attacks, downing helicopters and destroying drones and armoured vehicles.

It said Thursday evening that Azerbaijani forces had fired on two villages inside Armenia, close to Karabakh, killing one civilian.

Elbrus Babayev stands by a damaged car after ongoing fighting between Armenia and Azerbaijan in Nagorny Karabakh region. AFP

Armenian Deputy Prime Minister Tigran Avinyan said on Facebook that 1,280 Azerbaijani troops had been killed and 2,700 wounded since Sunday, as both sides have made claims of heavy casualties inflicted on their opponent.

In turn, Azerbaijan's defence ministry said its forces had carried out "crushing artillery strikes" on Armenian forces.

Azerbaijan denied claims by Yerevan that one of its helicopters was shot down and had crashed in Iran.

The two sides have accused each other of shelling civilian areas and ignored repeated calls from international leaders to halt the conflict that threatens to draw in regional powers Turkey and Russia.

Putin, Macron and Trump called for an "immediate cessation of hostilities" in their statement Thursday and urged the warring sides to commit to talks to resolve the fighting.

"We also call on the leaders of Armenia and Azerbaijan to commit without delay to resuming substantive negotiations," said the leaders, whose countries are the co-chairs of the OSCE Minsk group that has sought a solution to the conflict since the 1990s.