Speaking at an annual Paris gathering of ambassadors from around the world, Francois Hollande touched on crises in both countries as well as those rocking Ukraine and Libya -- all of which he said affected France directly.
"Assad cannot be a partner in the fight against terrorism, he is the de-facto ally of jihadists," he said.
Hollande reiterated his proposal to hold an international conference in Paris "to organise the coordination of international action against the Islamic State on humanitarian, security and military fronts".
The United States has already carried out reconnaissance flights on Islamic State positions in Syria in surveillance seen as a precursor to possible strikes against the radical group, whose campaign of extreme violence has terrorised civilians and rivals alike.
Hollande also touched on the crisis in Ukraine, where Western intelligence believes Russian army units are taking part in the 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.
Over the past weeks different factions in the country have backed rival prime ministers and national assemblies, while Egypt and the United Arab Emirates have carried out air strikes against Islamists.
At the weekend, Islamist fighters seized Tripoli airport, compounding a crisis that has been boiling since the fall of long-time dictator Moamer Kadhafi in 2011.
"France asks the United Nations... To organise exceptional support for Libyan authorities to restore their state," he said, without detailing what form this support should take.
