Barack Obama has said the US will formally recognise a Syrian opposition coalition as the de facto administration of regions under rebel control in a move that further saps the legitimacy of president Bashar al-Assad's rule.
The president's announcement, in an interview with ABC News, comes as Washington attempts to bolster support for rebel groups it regards as acceptable to western interests while attempting to isolate others.
Obama said the US has thrown its weight behind the coalition because he said it is open and representative enough to include various ethnic and religious groups.
"We've made a decision that the Syrian Opposition Coalition is now inclusive enough, is reflective and representative enough of the Syrian population, that we consider them the legitimate representative of the Syrian people in opposition to the Assad regime" he said.
But the president did not address whether the US is now prepared to arm the rebels or offer other military support similar to the air bombardment in support of Libya insurgents fighting Colonel Muammar Gadaffi nearly two years ago.
Obama's comments came before a meeting between senior US state department officials and Syrian opposition leaders, under the umbrella of the National Coalition of Syrian Revolutionary and Opposition Forces, in Marrakech, Morocco on Wednesday at which the recognition agreement is expected to be formalised.
Hours earlier, the Obama administration declared one Syrian resistance group, the al-Nusra Front for the People of the Levant, a terrorist organisation and an al-Qaida front.
"Not everybody who is participating on the ground in fighting Assad are people that we are comfortable with," said Obama. "There are some who I think have adopted an extremist agenda, an anti-US agenda."
The recognition of the coalition comes amid growing disillusionment among Syrian opposition fighters, some of whom have complained of being abandoned by the US when other countries, including Britain, France and Turkey, have already recognised the coalition. They have compared the lack of active American backing with the support given to rebels in Libya.
Washington expects to work with the Syrian opposition coalition to established its authority in areas now under rebel control, in part by working with local councils. That is intended not only to isolate extremist groups but to prepare the coalition to take national power if and when Assad is overthrown.