The United States has urged US citizens to depart Syria "immediately," and noted that the Damascus government has placed "severe" constraints on the travel of US diplomats in the country.

The travel warning, an update to an April 25 order for US embassy staff's family members and some non-emergency personnel to leave Syria, expressed growing concern over the violent crackdown by President Bashar al-Assad's security forces against pro-democracy protesters.

Last month, pro-government demonstrators attacked the US and French embassies in Damascus, smashing windows and spray-painting walls after the two Western envoys visited the flashpoint protest city of Hama.

"The US Department of State urges US citizens in Syria to depart immediately while commercial transportation is available," the latest travel warning said.

It said "Syrian government constraints on observers, including the short-term detention of accredited diplomats, have made it difficult for US embassy personnel to adequately assess the current risks or the potential for continuing violence."

Washington sent its ambassador back to Damascus on Thursday as it gradually cranked up the heat on Assad. Robert Ford had infuriated the Assad regime with his visit to Hama last month, but he has vowed to continue traveling around the country. French Ambassador Eric Chevallier also visited the city on July 7.

Syria responded by imposing a ban on the two envoys prohibiting them from traveling outside Damascus.

Noting that Syrian demonstrations have been "violently suppressed" since March, the State Department warned US citizens that "demonstrations can occur with little or no warning anytime and anywhere, not just on Friday afternoons, as with many past demonstrations."

"We remind US citizens that even demonstrations intended to be peaceful can turn confrontational and escalate into violence," it added. "US citizens are urged to avoid the areas of demonstrations if possible, and to exercise caution if within the vicinity of a demonstration."

"Syrian efforts to attribute the current civil unrest to external influences may lead to an increase in anti-foreigner sentiment," the State Department warned.

"Detained US citizens may find themselves subject to allegations of incitement or espionage."

It said Syrian authorities often do not notify the US embassy of a US citizen's arrest until days or weeks later, and decline requests for consular access.