The Obama administration on Wednesday renounced the proclaimed leaders of the Syrian political opposition and said any group seeking to oust President Bashar al-Assad must reject attempts by extremists to “hijack” a legitimate revolution.

Secretary of State Hillary Rodham Clinton said the Syrian National Council, or SNC, should no longer be considered the “visible leader” of the opposition. That made official what has been the increasingly obvious sidelining of an opposition group led mostly by middle-age Syrian expatriates.

“This cannot be an opposition represented by people who have many good attributes but have in many instances not been inside Syria for 20, 30 or 40 years,” Clinton said during a five-nation Balkans tour. “There has to be a representation of those who are on the front lines fighting and dying today.”

The United States has no direct power to anoint the would-be new leaders of Syria, but U.S. backing will be essential for any hopefuls seeking outside financial, diplomatic or possible military assistance. The United States is supporting new opposition leaders who will attend a strategy session in Qatar next week, Clinton said.

Clinton and other U.S. officials are fed up with infighting among the SNC leaders seeking recognition as a shadow government and have become convinced that the group does not represent the interests of all ethnic and religious groups in Syria. It also has little legitimacy among on-the-ground activists and fighters, and has done little to stem the infiltration of Islamist extremists into the opposition forces.

Clinton had some of her strongest words to date about the risk that the uprising against Assad could be overtaken by militants who do not seek a democratic replacement.

“We also need an opposition that will be on record strongly resisting the efforts by extremists to hijack the Syrian revolution,” Clinton said. “There are disturbing reports of extremists going into Syria and attempting to take over what has been a legitimate revolution against an oppressive regime for their own purposes.”

The United States and other Western and Arab nations have called on Assad to step down. Russia, China and Iran continue to back him.

Anti-regime activists say that in recent weeks, about 150 people a day have been killed in fighting.

The Obama administration’s shift away from the SNC comes as an internationally sanctioned truce failed to take hold. More than 500 people were killed during what was supposed to be a four-day cease-fire ending Monday.

Clinton said she was saddened but not surprised and made clear the United States is not betting on U.N.-sponsored diplomacy to end the fighting.

She said nothing about possible military intervention, something the United States has opposed as unworkable and counterproductive. Some Syrian activists hope that calculation will change after the U.S. presidential election next week, when President Obama would have more latitude to act, but there is no obvious sign of any change. Having a better-organized and more representative opposition to turn to would add legitimacy to any international military effort, however.

The State Department helped to smuggle a few internal dissidents out of Syria and brought them to New York last month to meet with representatives of key nations trying to help the opposition, Clinton said.

The larger meeting during the annual U.N. General Assembly was public at the time, but the presence of Syrian-resident dissidents was not. Clinton did not name the participants nor say whether they had returned to Syria.

“We made it clear that the SNC can no longer be viewed as the visible leader of the opposition,” Clinton said. “They can be part of a larger opposition, but that opposition must include people from inside Syria and others who have a legitimate voice.”

Clinton spoke in the Croatian capital, Zagreb, as she nears the close of a five-day trip.

Also Wednesday, Turkish Prime Minister Recep Tayyip Erdogan said it is up to the U.N. Security Council to decide whether a no-fly zone should be imposed on Syria or safe areas created for civilians fleeing the civil war.

Erdogan has been one of the sharpest critics of Assad’s violent crackdown on the opposition.

Activists say 35,000 people have been killed in Syria since an uprising against Assad’s regime began in March 2011.