Clinton condemned the Syrian attacks and said the US did not believe the long-time Syrian ruler would follow through on his promises to reform in the face of escalating protests against his rule.

"From our perspective, he has lost legitimacy, he has failed to deliver on the promises he's made, he has sought and accepted aid from the Iranians as to how to repress his own people," Clinton told reporters in an appearance with European Union foreign policy chief Catherine Ashton in Washington.

Clinton's comments marked a significant sharpening of US rhetoric on Assad, whose security forces have waged an increasingly brutal crackdown against protesters inspired by pro-democracy movements elsewhere in the Arab world.

Several Assad loyalists broke into the US embassy in Damascus on July 11 and security guards used live ammunition to prevent hundreds from storming the French embassy, Western diplomats in the Syrian capital said.

They said the attackers tore down US embassy plaques and tried to break security glass in protests fueled by the government against a visit by US and French ambassadors to the city of Hama, focus of protests against Assad's rule.

A French foreign ministry official said the Syrian authorities had done nothing to stop the assault.

France has led Western attempts to pass a United Nations Security Council resolution condemning Syria's hierarchy for cracking down on protesters. It says the president has lost legitimacy because of the number of killings to try to quell the protests demanding political freedoms after 41 years of Assad family rule.

The United States, which sees Syria as a fragile but crucial element of any lasting Middle East peace equation, had been reluctant to take that step, but Clinton's comments on July 11 indicated the US's patience had run out.