"We have made our views very clear: This is a red line for the United States," Clinton told reporters. "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 is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads.
Its arsenal is a particular threat to American allies Turkey and Israel, and Obama singled out the threat posed by the unconventional weapons earlier this year as a potential cause for deeper U.S. involvement in Syria's civil war. Up to now, the United States has opposed military intervention or providing arms support to Syria's rebels for fear of further militarizing a conflict that activists say has killed more than 40,000 people since March 2011.
Clinton said that while the actions of President Bashar Assad's government have been deplorable, chemical weapons would bring them to a new level.
"We once again issue a very strong warning to the Assad regime that their behavior is reprehensible, their actions against their own people have been tragic," she said. "But there is no doubt that there's a line between even the horrors that they've already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."