In Washington, a U.S. defense official said American and allied intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week. He spoke on condition of anonymity because he was not authorized to speak publicly about intelligence matters.
He said officials don't believe any developments with the weapons are imminent but are trying to figure out what the Syrians are doing.
More than 40,000 people have been killed since the Syrian revolt started in March last year.
Clinton said the actions of Assad's government have been deplorable, but the use of chemical weapons would bring them to a new level.
"We have made our views very clear: This is a red line for the United States," Clinton told reporters in Prague.
"I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people. But suffice it to say, we are certainly planning to take action if that eventuality were to occur."
Syria's Foreign Ministry denied Damascus has chemical weapons, and said in a statement to Syrian state TV that it would never use them against its own people, even if it had them.
Surk reported from Beirut. Associated Press writer Bradley Klapper in Prague and Pauline Jelinek in Washington contributed to this report.