Obama
ObamaReuters

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.

The United States has edged slowly towards recognizing the opposition Syrian National Council, and its move follows those of France, Britain, Turkey and the Gulf Cooperation Council regional grouping.

The process was slowed by concerns that the group, recently reconstituted under U.S. pressure, did not genuinely represent all sectors of Syrian society, had links to extremists, and did not fully subscribe to democratic principles.

"We have 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," Obama told ABC News in an interview.

Secretary of State Hillary Clinton had been expected to make the announcement at a Friends of the Syrian People meeting in Marrakesh, Morocco, Wednesday but could not travel owing to illness, AFP reported.

Washington has so far only provided humanitarian, non-lethal aid to the rebels, officially declining to send arms, a position White House spokesman Carney reiterated on Tuesday.

The U.S. administration made clear that it was differentiating between the Council and another group, Al-Nusra, which it sees as having extreme tendencies.

"There is a small element of those that oppose the Assad regime, that in fact are affiliated with Al-Qaeda in Iraq and we have designated them, Al-Nusra, as a terrorist organization," Obama said in the ABC interview.

Though a minority, Al-Nusra has been one of the most effective rebel groups fighting to overthrow Assad, raising concerns that hardline extremists are hijacking the 21-month-old revolt.

The front's fighters, many of them jihadist volunteers from around the Islamic world, were instrumental in the fall of the army's massive Sheikh Suleiman base in northern Syria on Monday after a months-long siege.

The group has claimed responsibility for recent suicide bombings that killed scores of people, and has said it hopes to replace the Assad family's four-decade hold on power with a strict Islamic state.

Washington also Tuesday said it was now less concerned than last week that Assad could resort to using chemical weapons stockpiles against rebels.

U.S. Defense Secretary Leon Panetta said Syria had not taken any new steps in recent days that signal a readiness to use its arsenal.

"At this point the intelligence has really kind of leveled off. We haven't seen anything new indicating any aggressive steps to move forward in that way," Panetta told reporters aboard his plane before landing in Kuwait, AFP reported.

A former top general in Syria's chemical weapons program said on Monday he doesn't doubt for a moment that Assad will deploy his chemical weapons arsenal as he tries to hold onto power.

"The regime started to fall and deteriorate. It's coming to its end," retired Major General Adnan Sillou told ABC News. "It's highly possible that he'll start using [chemical weapons] to kill his own people because this regime is a killer.”