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.