"We're not giving them a list," said State Department spokesman Mark Toner. "Ultimately it's up to the Syrians themselves to make those choices. This is in no way telling them what to do."
But Clinton's remarks were seen as damaging by opposition leaders and ordinary Syrians long wary of U.S. meddling in the region. The opposition has been increasingly frustrated by what it perceives as the lack of a coherent U.S. plan to help the rebels.
Muhydin Lazikani, a London-based writer and SNC member, said Clinton had no business criticizing the SNC at a time when the Obama administration has not charted a path for Syria.
"All they try to do is blame the SNC," said Lazikani.
Mohammad Sarmini, a Turkey-based SNC spokesman, said the U.S., through this new push, is "trying to make up for its shortcomings and impotence to stop the killings and massacres in Syria."
Shadi Hamid of the Brookings Doha Center was also critical of the U.S. approach.
"The U.S. does not seem to have a real end game here," he said. "Where does this lead? What happens after you have a unified opposition? It will still have to be fought out between armed groups."
The shift in the U.S. position came after months of fruitless attempts by the Obama administration and its allies to cajole the notoriously fractious SNC to broaden its base, according to two American officials.
The U.S. wants the SNC to include representatives of all Syria's diverse ethnic and religious groups as well as members of the armed opposition not affiliated with extremist groups or causes, said the officials, speaking on condition of anonymity because they were not authorized to discuss the plans publicly.
Those potential members, including opposition figure Riad Seif, are among hundreds of opposition figures that U.S. diplomats have been impressed with in discussions during the course of the crisis, the officials said. Seif is a former reformist lawmaker who was frequently jailed even before the uprising began.
Syrian opposition leaders confirmed that Seif was among the top candidates being considered to head a transitional government. Seif, who suffers from cancer, was beaten up by security forces at a protest in October last year before he finally left the country. He could not immediately be reached for comment Thursday.
In addition to a greater role for the Free Syrian Army, the U.S. is also pushing for more representation of local coordinating committees and the mayors of liberated cities who are already displaying skills at local leadership and governance, the officials said.
Some have accused the fundamentalist Brotherhood of dominating the Turkey- and Qatar-backed SNC and using it as a front. Different branches of the Brotherhood have been gaining power across the Middle East in Arab Spring uprising against dictatorships — particularly in Egypt where the group now holds the presidency and dominated in parliamentary elections.
Hundreds of Syrian opposition figures are expected to take part in the Doha conference. They will aim to choose a new leadership, said George Sabra, an SNC spokesman. More than 400 delegates are to select a 40-member general secretariat, a 15-member executive bureau and a new leader.
The conference will discuss the possibility of setting up a transitional government for Syria, but it is not expected to declare the formation of that body.
Associated Press writers Mathew Lee in Washington and Bradley Klapper in Shannon, Ireland, contributed to this report.