Assad's forces have intensified assaults on towns and cities across the country since the start of the Muslim fasting month of Ramadan to subdue mounting dissent against the ruling family, despite threats of new US sanctions and calls from Turkey, Syria's powerful northern neighbor, and Arab nations, to halt the attacks.

"Assad remains convinced that the military solution is working, ignoring the fact that as soon as he contains demonstrations in one town they erupt in another," a Western diplomat in the Syrian capital said.

Activists and rights campaigners said 11 civilians, including a woman and a child, were killed on Thursday when troops and tanks swept into Qusair, 135 km (85 miles) north of Damascus, after overnight protests calling for Assad's removal.

Syrian authorities have expelled most independent journalists since the five-month-old uprising began, making it difficult to verify reports from both sides.

Secretary of State Hillary Clinton said sanctions against Syria's oil and gas industry were needed to put pressure on Assad. She called on Europe and China, main foreign players in Syria's energy sector along with Russia, to do more in imposing sanctions on Syria.