President Barack Obama proclaimed Syria's newly reframed opposition as the "legitimate" representative of the nation's people, in the most significant US 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 is seeking to hijack the revolution, as a terrorist group.
Russia on Wednesday said it was surprised by Obama's recognition of the opposition, saying Washington was now betting on an armed victory by rebels in the conflict.
"I was somewhat surprised to find out" about the recognition announcement, Foreign Minister Sergei Lavrov said, quoted by Russian news agencies. "The United States has decided to place all its bets on an armed victory of the (Syrian) National Coalition."
It was another day of carnage inside Syria, meanwhile, as scores of civilians from Assad's minority sect were reported killed, in what appeared to be the largest scale revenge attacks yet against Alawites.
The United States has edged slowly towards recognizing the opposition Syrian National Coalition, and its move follows similar action by France, Britain, Turkey and the Gulf Cooperation Council regional grouping.
The process was slowed by concerns that the coalition, recently reconstituted under US pressure, did not represent all 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.
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 US administration made clear that it was differentiating between the Council and 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 interview.
Declaring Al-Nusra a terrorist group freezes any assets under US jurisdiction and bans Americans from any transactions with it. US officials also said the move would make sure aid reaches the right hands.
Countries wanting to support the opposition need to ensure they are helping "those opposition groups who truly have the best interest of Syria and Syrians in mind," State Department spokeswoman Victoria Nuland said.
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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.
Al-Nusra has also 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.
The strike on Alawites came in bomb attacks in the village of Aqrab in the central province of Hama and killed or wounded at least 125 civilians, said the Syrian Observatory for Human Rights.
"We cannot know whether the rebels were behind this attack, but if they were, this would be the largest-scale revenge attack against Alawites," members of a Shiite sect in Sunni-majority Syria, said Observatory director Rami Abdel Rahman.
Aqrab is near Houla, a majority Sunni Muslim village where 108 people, including 49 children and 34 women, were massacred on May 25 in what was widely blamed on pro-regime militias despite denials from Damascus.
Washington also Tuesday said it was now less concerned than last week that Assad could resort to using chemical weapons stockpiles against rebels.
US Defence 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.
International military chiefs have met in London to discuss the Syria conflict, a diplomatic source said after a media report that they discussed plans to train rebels and give air and naval support.
A British diplomatic source confirmed that the military leaders had held talks, but played down the idea that they discussed military intervention against the Assad regime.
"As far as I know they didn't explore options in any detail, certainly they didn't explore options for military intervention," the diplomat said on condition of anonymity.
Inside Syria, and apart from the Aqrab attack, at least 68 people were killed Tuesday, the Observatory said.
With the total death toll from Syria's agony now topping 42,000, according to the Observatory's figures, the UN High Commissioner for Refugees said the number of Syrian refugees in neighboring countries and the wider Arab world had now passed half a million.