U.S. President Barack Obama proclaimed Syria's newly reframed opposition as the "legitimate" representative of the nation's people on Tuesday, AFP reported, in the most significant U.S. intervention in a brutal civil war.

As Washington cranked up pressure on beleaguered President Bashar al-Assad, the Obama administration also blacklisted the Al-Qaeda-linked Al-Nusra Front, which officials fear seeks to hijack the revolution, as a terrorist group.