Riyadh (AFP) - Military pressure may be needed to oust Syria's President Bashar al-Assad, US Secretary of State John Kerry said in Saudi Arabia on Thursday.

"He's lost any semblance of legitimacy, but we have no higher priority than disrupting and defeating Daesh and other terror networks", he told reporters, using an Arabic acronym for the Islamic State group which has seized swathes of Syria and Iraq.

"Ultimately a combination of diplomacy and pressure will be needed to bring about a political transition. Military pressure particularly may be necessary given President Assad's reluctance to negotiate seriously."