The decision is the most dramatic U.S. move so far after 11 months of a violent crackdown by President Bashar Assad's regime.
Reports say for a third day, troops heavily shelled the city of Homs, the epicenter of the country's uprising. It's being called the fiercest attack yet. Activists say at least 17 people were killed.
The government denies shelling the city and claims armed terrorist groups attacked civilians and police.
The U.S. vowed to step up pressure on Assad to quit but ruled out military intervention.
On Saturday, Syrian forces killed up to 200 people in Homs - the highest death toll reported for a single day in the uprising - according to several rights groups. There was no way to confirm the toll because Syria has blocked access to trouble spots in the country and prevented independent reporting.
More than 5,400 people have been killed since the Arab Spring-inspired uprising began in March, according to the U.N.
The Associated Press contributed to this report.