US Secretary of State Hilary Clinton has urged Syria's opposition to resist efforts by extremist groups to "hijack the revolution".
Speaking on a trip to Croatia, she said the rebel leadership needed to be more inclusive of those fighting in Syria.
The Turkey-based Syrian National Council (SNC) has been powerless to stop the escalating violence.
Mrs Clinton also revealed the Obama administration has suggested who should feature in the rebel leadership.
"There has to be a representation of those who are in the front lines fighting and dying," she told reporters.
"This cannot be an opposition represented by people who have many good attributes but who, in many instances, have not been inside Syria for 20, 30, 40 years."
The secretary of state said there had been "disturbing reports" of Islamic extremists entering Syria to try and take advantage of the uprising against President Bashar al-Assad...