US Secretary of State Hillary Clinton demanded that Syria "take every possible step" to protect American diplomats after supporters of President Bashar al-Assad tried to attack the US ambassador.
The attempt to storm an office in Damascus where the ambassador, Robert Ford, had just arrived, came with the UN Security Council divided over whether to threaten Syria with sanctions.
Opposition figure Hassan Abdelazim, whom the US ambassador had arrived to meet, told AFP that the mob "tried to break down the door of my office, but didn't succeed" during a siege that lasted two hours.
In Washington, 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 also spoke of an "ongoing campaign of intimidation" against not only US diplomats but those from other countries.
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.
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.
Meanwhile divisions over whether to threaten sanctions against Syria for the government's deadly crackdown held up UN Security Council discussions on a resolution on the crisis Thursday.
European nations and Russia have proposed rival resolutions on Syria, where more than 2,700 people have died in the past seven months, according to the United Nations.
Britain, France, Germany and Portugal insist that any resolution must include at least the threat of sanctions against President Bashar al-Assad.
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"There are still divergences. We want to keep the essential message in the resolution: that is if repression and violence doesn't stop there will be further measures," Germany's UN ambassador Peter Wittig told reporters after the latest talks.
Russia opposes any mention of sanctions in the text. "And we are not the only ones in this position," said Vitaly Churkin, Russia's UN ambassador after the latest talks.
The Europeans had originally drawn up a text which set out firm sanctions against Assad. Russia and China have threatened to use their veto power, as permanent members of the council, against any resolution with punitive measures.
The two sides also disagree over whether violence by protesters should be given the same importance as that by the government. India's UN envoy Hardeep Singh Puri said there had to be a reference to violence by "extremist elements" in the text.
France was to draw up a new version of the European text for possible talks on Friday.
Angry mobs stormed the US 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.
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.
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.
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.