Syrian President Bashar al-Assad has lost legitimacy and is "not indispensable," U.S. Secretary of State Hillary Clinton said on Monday as tension soared over an assault by Assad loyalists on the U.S. and French embassies in Damascus.

Clinton condemned the Syrian attacks and said Washington 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 U.S. 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 U.S. embassy in Damascus on Monday 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 U.S. embassy plaques and tried to break security glass in protests fuelled by the government against a visit by U.S. and French ambassadors to the city of Hama, focus of protests against Assad's rule.

One of the diplomats said: "This is a violent escalation by the regime. You do not bring busloads of thugs into central Damascus from the coast without its consent."

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

"(France) reminds (Syria) that it is not with such illegal methods that the authorities in Damascus will turn the attention away from the fundamental problem, which is to stop the repression of the Syrian population and to launch democratic reform," foreign ministry spokesman Bernard Valero said.

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.

"Four buses full of shabbiha (Alawite militia loyal to Assad) came from Tartous. They used a battering ram to try to break into the main door," a resident of Afif, the old district where the French embassy is located, told Reuters by telephone.

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 Monday indicated Washington's patience had run out.

"If anyone, including President Assad, thinks that the United States is secretly hoping that the regime will emerge from this turmoil to continue its brutality and repression, they are wrong," Clinton said.

"President Assad is not indispensable and we have absolutely nothing invested in him remaining in power."

The United States condemned Syria for "refusing" to protect the embassy from an assault it said had been encouraged by a pro-government television station, and called in a senior Syrian diplomat to deliver a formal complaint.

'Shadow government'

Meanwhile, a prominent Syrian dissident says the country's opposition will form a shadow government.

Speaking in Turkey, the former judge Haitham Al-Maleh said the aim of this shadow government was to help steer the country towards free elections and a new constitution.

"Each shadow minister will have a portfolio and a team of people working on specific goals," Haitham Al-Maleh said.

"All the Syrian people know that Assad's regime will eventually collapse and therefore we must prepare ourselves."

Agencies