Obama rules out alliance with Syria's Assad against IS

US President Barack Obama on Sunday rejected any alliance of convenience with Bashar al-Assad against the Islamic State extremists, arguing that the Syrian ruler had "lost legitimacy" with his people.

Barack Obama speaks at his closing press conference of the G20 summit. (AAP)

"For us to make common cause with him against ISIL (Islamic State) ... would weaken our coalition," Obama told reporters after a G20 summit in Brisbane.

MORE TO COME.