The LCC called on rebels to deliver a knockout punch to the regime while protecting civilians, religious sites and the nation's cultural heritage.

It also urged rebels to preserve any documents found in offices of state security services seized by rebels in preparation for possible war crimes trials.

"These documents contain massive amounts of incriminating evidence against the regime and its symbols and will be required to hold the regime accountable, compensate victims and retain a historical record of decades of state behavior," the group said.

State Department spokeswoman Victoria Nuland said U.S. government-trained medical workers have reached an estimated 410,000 patients in Syria, performed 22,370 surgeries and had partnered with a group -- which she would not identify for security reasons -- that has set up 20 field hospitals in the country.

"I don't think any of us has a crystal ball as to exactly how this is going to go, but we do believe that the Assad regime's days are numbered," Nuland said. "The opposition in recent days and weeks has made a number of significant captures, in particular major military facility outside of Aleppo with the last Sheik Suleiman base and other important military installations."

She urged Russian officials to withdraw support for the al-Assad regime.