Activists said the army had fired guns, tanks, and anti-aircraft weapons on residents in the central city.

Syria has not commented on the reports and it is hard for international media to verify such statements.

Meanwhile, the US has ramped up pressure on Syria by imposing sanctions on a bank and mobile phone operator.

"We are taking aim at the financial infrastructure that is helping provide support to Syrian President Bashar al- Assad and his regime's illicit activities," said Under-Secretary for Terrorism and Financial Intelligence David Cohen.

The UN Security Council was due to meet on Wednesday evening to debate the crackdown. Last week, it condemned the violence and called for those responsible to be held "accountable".