Clinton told CNN in Geneva, Switzerland, even with China and Russia on board with the U.N. plan agreed to on Saturday, there remains the likelihood the ongoing rebellion in Syria would continue rather than move into a transitional-government phase.
"There is no guarantee that we are going to be successful," Clinton said. "I just hate to say that."
Clinton said success would hinge on Assad's key allies, Russia and China, convincing him and his inner circle to step down. "They have committed to trying, but they also admitted that they may or may not have enough leverage to convince not just one man, but a family and a regime that their time is over," Clinton said.
Anti-Assad forces have also thus far rejected the plan, which Clinton said left the likelihood Assad would remain in power rather than stepping aside in favor of a transitional government. The rebels said the plan was too vague to guarantee Assad's ouster; however, Clinton insisted Assad and his inner circle would not be part of the new government.
Meanwhile, at least 20 civilians and anti-government protesters were killed Sunday by the Syrian military, opposition activists said.
The Local Coordination Committees of Syria told CNN among those who died of injuries was a doctor tending to people wounded by an artillery blast at a funeral in Zamalka Saturday.
After the shelling, troops reportedly began shooting into the crowd of mourners.
Assad's regime has banned almost all international journalists since the uprising began 16 months ago, so opposition committees are nearly the sole source of news from Syria.
The group said since March 2011, more than 14,000 people have been killed by the military. In the past two months, the numbers have soared from 1,196 in May to 2,386 in June, the report said.