Clinton also said Syrian President Bashar al-Assad, reported to have accepted former UN Secretary-General Kofi Annan's peace plan, would be judged by his actions and must order his forces to stop firing and to withdraw from populated areas.
Assad has been using the army to crush efforts to end his family's four decades of political dominance in Syria. Disunity among the opposition has accentuated fears that Syria could slide into sectarian and ethnic conflict, much as Iraq did after the 2003 U.S.-led invasion that toppled Saddam Hussein.
Speaking ahead of a gathering of Western and Arab nations in Istanbul to discuss a political transition in Syria, Clinton strongly pressed the Syrian opposition to lay out a vision of an inclusive Syria in which minority rights are respected.
"They must be able to clearly demonstrate a commitment to including all Syrians and protecting the rights of all Syrians," Clinton told reporters. "We are going to be pushing them very hard to present such a vision in Istanbul. So, we have a lot of work to do between now and Sunday."
She also reacted skeptically to reports that Assad has accepted Annan's peace plan.
"Given Assad's history of over-promising and under-delivering, that commitment must now be matched by immediate actions," she said. "We will judge Assad's sincerity and seriousness by what he does, not by what he says."
"If he is ready to bring this dark chapter in Syria's history to a close he can prove it by immediately ordering regime forces to stop firing and begin withdrawing from populated areas," she said, repeating previous U.S. demands for Assad to allow in humanitarian groups, to release political prisoners and to begin a process toward a democratic transition.