The Obama administration plans to file a formal complaint with Syria over an attack Monday on the American Embassy in Damascus, where U.S. Marines forced back a mob after protesters breached the wall of the compound. A U.S. official told Fox News the protesters, who support President Bashar Assad, used a battering ram.

The latest incident comes after a 31-hour protest outside the embassy Friday and Saturday. The American ambassador in Syria expressed "U.S. displeasure" with that demonstration during a meeting over the weekend with Syria's foreign minister.

According to a State Department official, the U.S. now plans to lodge a formal complaint with a top Syrian diplomat in Washington. The central concern, the official said, is that Syrian security forces were slow to respond to the mob on Monday. Those forces eventually formed a perimeter to keep demonstrators out but only after U.S. Marines pushed back the protesters.

The protesters did some damage to the chancery building itself but no embassy staff were injured, according to the official. Protesters later moved on to attack the ambassador's residence.

State Department spokeswoman Victoria Nuland suggested Monday that the Syrian government was instigating the demonstrations while failing to provide proper security.

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"It's absolutely outrageous," she 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. And it's dangerous for our personnel. It's a violation of the Vienna Convention. ... They had a responsibility to protect the embassy. They've been repeatedly asked to do so, and they've repeatedly failed to do so."

Witnesses said the protesters smashed windows and raised a Syrian flag on the embassy compound. They also wrote anti-U.S. graffiti referring to the U.S. ambassador as a "dog," the witnesses said. The protests were over visits by the U.S. and French ambassadors last week to the opposition stronghold of Hama in central Syria.

On Sunday, the State Department complained that pro-government demonstrators threw tomatoes, eggs and rocks at the embassy over the weekend to protest Ambassador Robert Ford's visit to Hama. There were no reports of injuries, but a senior department official said two embassy employees were pelted with food during the 31-hour demonstration.

Ford on Thursday visited Hama where he was greeted by friendly crowds who put flowers on his windshield and olive branches on his car, chanting, "Down with the regime!" The State Department said Ford made the trip to express support for the right of Syrian people to demonstrate peacefully.

The Syrian government denounced Ford's visit, saying the unauthorized trip was proof that Washington was inciting violence in the Arab nation. The main headline of state-run daily Al-Thawra read, "Ford in Hama and Syrians are angry."

The Obama administration has criticized Assad's government for its violent crackdown on peaceful protests against his 11-year rule. Clashes between protesters and Assad's supporters have resulted in the deaths of 1,600, in addition to 350 members of the security forces.

But the White House has so far refrained from calling for an end to the Assad family's four decades of rule, leery of pressing too hard as it tries to wind down wars in Iraq and Afghanistan and faces criticism for being part of the coalition battling Muammar al-Qaddafi in Libya.

Congressional Republicans have pressed the administration to withdraw Ford from Syria, an ally of Iran that supports the Islamic militant groups Hezbollah in neighboring Lebanon and Hamas in the Gaza Strip. The U.S. did not send an ambassador to Damascus for five years in protest of Syria's alleged role in the assassination of a political leader in Lebanon.

The Associated Press contributed to this report.