WASHINGTON — Secretary of State Hillary Clinton hit back Monday after Syrian "thugs" besieged the US embassy in Damascus, saying President Bashar Al-Assad was "not indispensable" and had "lost legitimacy."

Syria's charge d'affaires was summoned to the foreign ministry in Washington to explain the Assad regime's "outrageous" failure to meet its international obligations to protect foreign embassies.

"President Assad is not indispensable and we have absolutely nothing invested in him... remaining in power," Clinton said. "From our perspective, he has lost legitimacy. Our goal is to see that the will of the Syrian people for a democratic transformation occurs."

Angry mobs besieged the US and French embassies Monday in apparent retaliation for alleged interference in Syrian affairs by the countries' ambassadors, who last week traveled to the flashpoint protest city of Hama.

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.

US officials have accused Assad's regime of orchestrating protests at foreign embassies for propaganda purposes and to punish the French and American ambassadors for visiting Hama.

After a mob stormed the US embassy for the second time in three days, the State Department said the Syrian charge d'affaires Mounir Koudmani had been summoned in place of Ambassador Imad Moustapha, who was on vacation.

It was made "clear that we consider that the Syrian government has not lived up to its obligations under the Vienna Convention to protect diplomatic facilities. And it's absolutely outrageous," spokeswoman Victoria Nuland said.

Nuland defended Ambassador Robert Ford's visit last Thursday to Hama, saying "we categorically reject" accusations from Assad's regime that it was designed to instigate disturbances.

"The ambassador's trip to Hama made clear to average Syrians -- not just in Hama, but around the world -- that we stand with those who are seeking their universal human rights to stand up and be counted, to ask for change in a peaceful manner," she said.

"This is all about the Syrian government being unwilling to live up to what's really going, to understand what's really going on in its country, that its own people want change," Nuland said.

"This is not about foreign governments. This is not about outside instigators. This is not about gangs of thugs. This is about average Syrians wanting change."

Ford complained to Syrian Foreign Minister Walid Muallem on Sunday about the lack of protection at the US embassy after a first round of food-throwing and rowdy protests Friday-Saturday.

"In that meeting, Ambassador Ford made clear that we did not think that the Syrian government was doing a good enough job in preventing incidents around the embassy," Nuland said.

"And in that meeting, it's my understanding, the foreign minister pledged to do a better job. So no sooner does he make that pledge when today we have thugs going over the walls."

The chancery was not breached but some of the crowd of around 300 climbed up on the roof and reached the ambassador's residence before being chased away by US Marines guarding the mission. Windows were broken, cameras smashed, walls spray-painted, but no one was injured, officials said.

"I think our main concern here is that the Syrian government, rather than dealing with its own internal problems and rather than addressing the grievances of its own people, is seeking to make distractions around our embassy," Nuland said.

"We are concerned that a government-sponsored TV station seemed to be inciting this mob earlier today."

Ford and French counterpart Eric Chevallier visited Hama on Thursday amid fears of a bloody protest crackdown by Assad's forces, with tanks encircling the city.

Syrian state news agency SANA said both envoys were summoned Sunday to the foreign ministry to protest the Hama trip. A senior US official in Washington said Ford's meeting was already scheduled, at the American's request.