Syria's Bashar al-Assad is not the West's partner in the fight against terrorism but an ally of Islamic extremists wreaking havoc in Syria and Iraq, French President Francois Hollande warned Thursday.

"Assad cannot be a partner in the fight against terrorism, he is the de facto ally of jihadists," he told a Paris gathering of ambassadors from around the world.

His comments came after Assad's regime said on Monday it was willing to work with the international community, including Washington, to tackle extremist fighters in the war-ravaged country.

Hollande also touched on other crises around the world, including Ukraine where Western intelligence believes Russian army units are operating as part of a four-month conflict that has seen more than 2,200 people lose their lives.

"If it turns out that there are Russian soldiers present on Ukrainian soil, it would be intolerable and unacceptable," he said.