Angry supporters of President Bashar al-Assad tried Thursday to attack the US ambassador to Syria, in what Washington charged was a government campaign to intimidate American diplomats.
A mob of nearly 100 Syrians chanting hostile slogans tried to storm an office in Damascus where the ambassador, Robert Ford, had just arrived to meet opposition figure Hassan Abdelazim.
Abdelazim told AFP the mob "tried to break down the door of my office, but didn't succeed" during a siege that lasted two hours.
Amid rising US-Syrian tensions, Damascus earlier accused Washington of inciting "armed groups" into violence against its army, which is trying to crush a six-month, pro-democracy movement.
In Washington, US Secretary of State Hillary Clinton said the United States has raised the attempted attack on Ford at "the highest levels" in Damascus and demanded that Syria "take every possible step to protect" US diplomats.
Clinton's deputy spokesman Mark Toner said the mob tried to attack Ford and other embassy staff while they visited the opposition leader, seriously damaging US vehicles and "pelting" the visitors with tomatoes.
However, he told reporters neither Ford nor other staff were hurt in the attack and all returned safely to the embassy after Syrian security officers finally came to their aid and cleared a path out of the building.
Toner charged that Assad's regime was behind the incident in what he said amounts to a campaign aimed at intimidating US diplomats as they carry out their duties.
Clinton spoke of an "ongoing campaign of intimidation" against not only US diplomats but those from other countries.
Angry mobs stormed the American and French embassies in Damascus on July 11 after Ford and the French ambassador visited the central city of Hama, a flashpoint for protests against Assad's regime.
White House spokesman Jay Carney said the attack on Ford was "unjustifiable" and "unwarranted" and called on the Senate to confirm him to allow his "courageous" work to go on.
Ford, the first US ambassador to Syria in more than five years, temporarily took up his post in January, but he is still awaiting Senate confirmation.
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The Syrian government meanwhile hardened its tone against the United States.
"Comments by American officials, notably Mark Toner, are striking proof that the United States encourages armed groups to commit violence against the Syrian Arab army," the foreign ministry in Damascus said.
"The words of the State Department spokesman, describing these terrorist acts as natural, are irresponsible and likely to encourage acts of terrorism and chaos in order to serve foreign goals against the interests of Syrians.
"Syria condemns strongly the US statements and affirms its determination to preserve its security and stability, to defend its citizens and to oppose all attempts to interfere in its internal affairs," said the statement.
Toner had told reporters in Washington on Monday that it came as no surprise that some arms are being sent to Syria's opposition who, he said, turn to violence as "an act of self-preservation" against the bloody crackdown.
He said the opposition had "shown extraordinary restraint in the face of the regime's brutality" and it would be "unfortunately, a natural development" for it to turn violent the longer its members are jailed and killed.
Since mid-March, Syria has been shaken by an unprecedented pro-democracy protest movement that the Assad regime has sought to crush using deadly force.
More than 2,700 people have been killed in the unrest, according to the UN High Commissioner for Human Rights in Geneva.
On the political front, prominent dissident Michel Kilo said anti-regime forces inside Syria oppose the Syrian National Council, an opposition body formed in Turkey last month, because it favours foreign intervention.
"If the idea of foreign intervention is accepted, we will head towards a pro-American Syria and not towards a free and sovereign state," he told AFP.
"A request for foreign intervention would aggravate the problem because Syria would descend into armed violence and confessionalism, while we at home are opposed to that."
And diplomats in Damascus said Ankara asked Syria this summer to offer the banned Muslim Brotherhood government posts in exchange for Turkey's support in ending the unrest, an offer rejected by Assad.