WASHINGTON (AP) -- U.S. and allied intelligence have detected Syrian movement of chemical weapons components in recent days, a senior U.S. defense official said Monday, as the Obama administration again warned the Assad regime against using them. A senior defense official said intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week. The defense official spoke on condition of anonymity because he was not authorized to speak publicly about intelligence matters. Secretary of State Hillary Rodham Clinton, in Prague for meetings with Czech officials, reiterated President Barack Obama's declaration that Syrian action on chemical weapons was a "red line" for the United States that would prompt action. "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." She didn't address the issue of the fresh activity at Syrian chemical weapons depots, but insisted that Washington would address any threat that arises. An administration official said the trigger for U.S. action of some kind is the use of chemical weapons or movement with the intent to use or provide them to a terrorist group like Hezbollah. The U.S. is trying to determine whether the recent movement detected in Syria falls into any of those categories, the official said. The administration official was speaking on condition of anonymity because they were not authorized to speak publicly about the issue. The senior defense official said the U.S. does not believe that any Syrian action beyond the movement of components is imminent. Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads. Its arsenal is a particular threat to the 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." Activity has been detected before at Syrian weapons sites, believed to number several dozen. Defense Secretary Leon Panetta said in late September the intelligence suggested the Syrian government had moved some of its chemical weapons in order to protect them. He said the U.S. believed that the main sites remained secure. Lawmakers said they were concerned. "I can't comment on these reports but I have been very concerned for some time now about Syria's stockpiles of chemical weapons and its stocks of advanced conventional weapons like shoulder-launched anti-aircraft missiles," said House intelligence committee Chairman Mike Rogers, R-Mich. "We are not doing enough to prepare for the collapse of the Assad regime, and the dangerous vacuum it will create. Use of chemical weapons by the Assad regime would be an extremely serious escalation that would demand decisive action from the rest of the world," he added. Syria is believed to have one of the world's largest chemical weapons programs, and the Assad regime has said it might use the weapons against external threats, though not against Syrians. The U.S. and Jordan share the same concern about Syria's chemical and biological weapons - that they could fall into the wrong hands should the regime in Syria collapse and lose control of them.