The State Department advised those who must stay behind against nonessential travel within the country. "US citizens not in Syria should defer all travel to Syria at this time," the agency said in an updated travel warning.

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

On April 25, the department ordered all eligible family members of US government employees and certain non-emergency personnel to leave Syria.

Syria has been beset by a six-month-old unrest it blames on foreign conspiracy and foreign thugs. The US slapped additional sanctions on Syria on August 18, targeting its oil and gas industry and freezing all of the government's assets. In concert with US' European allies, President Barack Obama also explicitly called on Syrian President Bashar Assad to step down.