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In a diplomatic shift, President Obama said today his administration now formally recognizes the newly-formed, leading coalition of Syrian rebels who are fighting to topple Syria's embattled President Bashar Assad .

"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," Obama said.

The announcement, made during an exclusive interview with ABC News' Barbara Walters, grants new legitimacy to the rebel group and marks a new phase in U.S. efforts to isolate the Assad regime.

"It's a big step," Obama said of the decision. The United States follows Britain and the European Union, both of which last month recognized the Syrian opposition group.

More of Barbara Walters' exclusive first joint, post-election interview with President Obama and first lady Michelle Obama airs Friday, Dec. 14, on "20/20" at 10 p.m. ET on ABC stations.

The diplomatic designation will allow the United States to more closely support rebel efforts, including the organization of a future post-Assad government, administration officials said.

"Obviously, with that recognition comes responsibilities," Obama said of the young coalition. "To make sure that they organize themselves effectively, that they are representative of all the parties, [and] that they commit themselves to a political transition that respects women's rights and minority rights."

The move does not include the provision of weapons , but it opens the door for that possibility in the future.

"Providing arms has to be done in a way that helps promote a political solution," one senior Obama administration official said today. "And until we understand how these arms promote a political solution, we do not see how provision of arms is a good idea."

But the official added, "the president has never ruled out in the future providing arms."

Obama expressed caution today about some Syrian factions involved with the coalition, warning that the United States will not support extremist elements.

"Not everybody who's participating on the ground in fighting Assad are people who we are comfortable with," Obama told Walters. "There are some who, I think, have adopted an extremist agenda, an anti-U.S. agenda, and we are going to make clear to distinguish between those elements."

The president specifically singled out the group Jabhat al-Nusrah for its alleged affiliation with Al Qaeda in Iraq. The State Department says the jihadist group is responsible for nearly 600 violent attacks in major Syrian cities in the past year.

"Through these attacks, al-Nusrah has sought to portray itself as part of the legitimate Syrian opposition while it is, in fact, an attempt by [Al Qaeda in Iraq] to hijack the struggles of the Syrian people for its own malign purposes," State Department spokeswoman Victoria Nuland said.

The Obama administration blacklisted al-Nusrah earlier this week, imposing economic sanctions and branding it a terrorist organization.

Recognition of the Syrian rebel group has been expected. Secretary of State Hillary Clinton was to formally announce the new relations with the United States during a meeting of international allies supporting Syria's rebels in Marrakech, Morocco, on Wednesday.

She has since cancelled her trip because of an illness. Her deputy, Bill Burns, will attend in her place.

President Obama also discussed the looming "fiscal cliff" and suggested a new flexibility on cuts to entitlement spending. Read that report here .

ABC News' Martha Raddatz contributed to this report.

More of Barbara Walters' exclusive first joint, post-election interview with President Obama and first lady Michelle Obama airs Friday, Dec. 14, on "20/20" at 10 p.m. ET on ABC stations.

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