US Secretary of State Hillary Clinton said Syrian leader Bashar Al-Assad had "lost legitimacy" after loyalists attacked the US and French embassies for alleged meddling in internal affairs.
Angry mobs besieged the US and French missions Monday after the countries' ambassadors last week travelled to the flashpoint protest city of Hama.
"President Assad is not indispensable and we have absolutely nothing invested in him... remaining in power," Clinton said in a significant sharpening of tone on Assad, whose forces have launched a brutal crackdown on protestors demanding a regime change.
"From our perspective, he has lost legitimacy. Our goal is to see that the will of the Syrian people for a democratic transformation occurs," she added.
The foreign ministry in Paris said three French staff were wounded in the embassy attack which forced guards to fire three warning shots, while a US official said "no staff were injured."
As Syrian security forces looked on, Assad supporters smashed their way into the French embassy compound with a battering ram, broke windows and destroyed the ambassador's car, according to a spokesman in Paris.
Several windows in the French mission were broken and Syrian flags were raised, one of them smashed through the windshield of a car belonging to an embassy staffer along with a portrait of Assad.
A crowd of people also attacked the nearby US embassy, several of them scaling the complex's high outer wall and others draping a large Syrian flag over the main entrance.
The chancery was not breached but some of the crowd of about 300 climbed up on the roof and reached ambassador Robert Ford's residence before being chased away by US Marines, US officials said.
Windows were broken, cameras smashed, and walls spray-painted, they said.
Monday's embassy attacks came four days after Ford and his French counterpart Eric Chevallier visited the central city of Hama, 210 kilometres (130 miles) north of Damascus, sparking outrage in the capital.
Signup to our newsletter and follow us on Facebook and Twitter!
Their visits took place amid fears of a bloody crackdown after Friday prayers by Assad's forces, with tanks encircling the city.
"We were subjected to an attack lasting three and a half hours, by people who had nothing to do with the demonstrators but seemed well prepared and who rammed the embassy and the residence," Chevallier said.
The assailants threw stones, "breaking almost all the embassy windows," and tried to get inside, he told the France 2 channel in Paris by telephone.
Ford, in a US embassy Facebook post dated Sunday morning and referring to a protest outside the embassy on Saturday, said demonstrators "resorted to violence, unlike the people in Hama, who have stayed peaceful."
"And how ironic that the Syrian government lets an anti-US demonstration proceed freely while their security thugs beat down olive branch-carrying peaceful protesters elsewhere," he wrote.
Tensions have been escalating sharply between Damascus and Washington over the Syrian government's fierce response to pro-democracy protests. Activists say 1,300 civilians have been killed and 12,000 arrested since mid-March.
"By either allowing or inciting this kind of behavior by these mobs against Americans and French diplomats and their property, they are clearly trying to deflect attention from their crackdown internally and to move the world's view away from what they're doing," said Clinton. "It just doesn't work."
Rights activists said protests were staged overnight in several towns against Sunday's opening of a "national dialogue," hailed by the regime but boycotted by the opposition.
Some 5,000 people demonstrated in Deir Ezzor in the east, the London-based Syrian Observatory for Human Rights reported, adding there were also protests in three Damascus districts.
The army was also reported to be conducting a search and sweep operation in the Jebel al-Zawiya area of Idlib province in the northwest.
People were detained in Hama and in the coastal city of Banias.
Sunday's inaugural "national dialogue" session saw some 200 delegates take part, including independent MPs and members of the Baath party, in power since 1963.
Opposition figures boycotted the gathering in protest at the ongoing deadly crackdown on anti-regime protests.