Syria's military forces continued their crackdown against anti-government protests on Thursday, shelling southern and central towns as Secretary of State Hillary Clinton warned President Bashar al-Assad that his legitimacy had nearly run out.

Syrian forces killed at least 13 civilians in the central town of Rastan on Thursday, activists said, in the latest military assault to try to crush a revolt against Assad's 11-year rule, Reuters reported.

Clinton noted that while the United States and its European allies have imposed sanctions on Syria, there seems to be a reluctance of other U.N. Security Council members, mainly China and Russia, to take more action.

Right now, the attitude of the international community is not as united as we are seeking to make it, she said. We do not yet have the agreement by some of the other members of the Security Council.

Russia and China so far have objected. China even warned that a European-proposed draft Security Council resolution that would condemn Syria for its crackdown would be a resolution that could harm stability in Syria, Reuters reported.

Those who we are seeking to bring to our view of the situation I think will have to make their own judgment, but we think they will be better off on the right side of history, Clinton said.

She also implied that U.S. patience with Assad is exhausted.

The legitimacy that is necessary for anyone to expect change to occur under this current government is, if not gone, nearly run out, she said.