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 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.

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

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

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.