The Obama administration has imposed sanctions on Syria's largest bank and cell phone operator.
Wednesday move was an effort to increase the pressure on Syrian President Bashar Assad.
The sanctions freeze any assets the companies have in the United States. They also bar Americans from doing business with the companies.
The administration is preparing for the first time to call for Assad to step down over his violent crackdown on political opponents.
Since March, more than 1,700 people have died in the crackdown.