France accuses Turkey's ally of starting recent warfare

"We have information, today, of a precise nature, that Syrian fighters have abandoned the battlefield, fighters from jihadist groups, and transited via Gaziantep [southern Turkey] to join the theatre of operations in Karabakh. This is very serious," he also said.

Macron spoke after four days of intense fighting between Armenia and Azerbaijan over the Armenia-occupied Nagorno-Karabakh region.

Both sides had accused each other of firing first on Sunday, with no independent monitors to verify claims.

Macron also gave weight to Armenian accusations that Azerbaijan's ally, Turkey, had sent Syrian mercenaries to Azerbaijan, despite Turkey's denials.

France is leading EU diplomacy on the conflict in its role as co-president, with Russia and the US, of the Minsk Group, a diplomatic forum for peace talks.

But Macron's comments were likely to turn up the heat between France and Turkey, who were already at odds over Turkey's military intervention in Libya and its gas drilling in Cypriot and Greek waters.

And for his part, Turkish president Recep Tayyip Erdoğan said, also on Thursday, that France and the other Minsk Group co-chairs had lost legitimacy "because they have neglected this problem for nearly 30 years".

Meanwhile, Cyprus came to the EU summit looking for sanctions on Turkey over its gas drilling.

"What I expect from the European Council summit is a more concrete and effective stance, to end this gunboat diplomacy," Cyprus president Nicos Anastasiades said on Thursday.

Austria and Greece also spoke out in hawkish tones.

"The European Union finally has to show president Erdoğan where our red lines are. This means the termination of enlargement talks, and sanctions," Austrian chancellor Sebastian Kurz said.

But Germany held out hope of a more amicable outcome.

The German EU president was "committed to finding a peaceful solution", German chancellor Angela Merkel said.

Cyprus had earlier vetoed EU sanctions on Belarusian officials over police brutality and election-rigging in order to force a joint move on Turkey.

But Lithuania and Poland expected EU leaders to break the impasse on Thursday.

And one possible solution was to promise Cyprus hard-hitting Turkey measures down the line if Erdoğan did not back off, in return for dropping the Belarus veto, diplomats told the Reuters news agency.

"People are being detained, tortured ... this is just inappropriate to have [such] a situation without any [EU] reaction," Lithuanian president Gitanas Nausėda said on Belarus.