ZAGREB, Croatia - The Obama administration 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 could be overtaken by extremists 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."

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