"It is starting to be moved as we speak," Pentagon spokesman Rear Admiral John Kirby told reporters.
The news followed an announcement by the joint mission of the United Nations and the Organization for the Prohibition of Chemicals that Syria had destroyed its entire declared stockpile of isopropanol, a precursor for producing sarin nerve gas.
"Now 7.2 percent of Syria's chemical weapons material remains in country and awaits swift removal for onward destruction. The Joint Mission urges the Syrian authorities to undertake this task as soon as possible," the U.N.-OPCW mission said in a statement.
Syria has promised to hand over or destroy its entire chemical weapons arsenal although it still possesses a significant amount of its declared chemical stocks and has not yet destroyed a dozen production and storage facilities.
A U.S. defense official, speaking on condition of anonymity, said Syrian President Bashar al-Assad's forces had taken steps to prepare some of its remaining chemicals for transport.
Syria has been removing chemical weapons under a deal reached last year that averted Western military strikes threatened after a sarin gas attack on rebel-held suburbs around the Syrian capital in August.
Assad's Western foes suspect him of deliberately dragging out the process and say his forces are using chlorine bombs, including in an attack on a rebel-held village this week.
Damascus denies that forces loyal to Assad have used chlorine or other more poisonous gases and blames all chemical attacks on rebel forces fighting in the three-year-old civil war. It has also blamed the security situation for its failure to meet deadlines to ship out the chemical stocks.
Syria did not declare chlorine as part of its stockpile.
Chlorine is thousands of times less lethal than sarin but is illegal under a chemical weapons convention that Syria signed and its use would breach the terms of the deal with Washington and Moscow.
Britain, France and the United States say they have intelligence suggesting Syria failed to declare some of its chemical weapons materials.