The Syrian government began to move its chemical weapons in recent days, senior U.S. officials confirmed to Fox News, prompting new warnings from President Obama and other top administration officials that using those weapons against the population would cross a "red line."
One senior U.S. official told Fox News "there are concerns about possible preparations for use" of the weapons, though "we don't know yet if they plan to use them."
The official added, "There are troubling signs of late."
Obama on Monday afternoon issued a stern warning to Syrian President Bashar Assad and his officials.
“I want to make it absolutely clear to Assad and those under his command: The world is watching,” the president said in a speech at the National Defense University, in Washington. “The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable."
His statement followed Secretary of State Hillary Clinton and White House Press Secretary Jay Carney issuing warnings earlier in the day.
"As the opposition makes strategic advances and grows in strength, the Assad regime has been unable to halt the opposition's progress through conventional means," Carney said. "And we are concerned that an increasingly beleaguered regime -- having found its escalation of violence through conventional means inadequate -- might be considering the use of chemical weapons against the Syrian people."
Carney said using or proliferating those weapons "would cross a red line for the United States."
"They will be held accountable by the United States and the international community if they use chemical weapons or fail to meet their obligation to secure them."
Carney would not say whether the movement of weapons by itself could cross the so-called red line.
Clinton, in Prague for meetings with Czech officials, also reiterated Obama's earlier declaration that Syrian action on chemical weapons was a "red line" for the United States that would prompt action.
"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 said.
Syria said Monday it would not use chemical weapons against its own people. The Ministry of Foreign Affairs said Syria "would not use chemical weapons -- if there are any -- against its own people under any circumstances."
Syria has been careful never to confirm that it has any chemical weapons.
The use of chemical weapons would be a major escalation in Assad's crackdown on his foes and would draw international condemnation. In addition to causing mass deaths and horrific injuries to survivors, the regime's willingness to use them would alarm much of the region, particularly neighboring states, including Israel.
Although Syria is one of only seven nations that have not signed the Chemical Weapons Treaty, it is a party to the 1925 Geneva Protocol that bans the use of chemical weapons in war. That treaty was signed in the aftermath of World War I, when the effects of the use of mustard gas and other chemical agents outraged much of the world.
Clinton 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 told the Associated Press that 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.
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.
Israeli officials have repeatedly expressed concerns that Syrian chemical weapons could slip into the hands of Hezbollah or other anti-Israel groups, or even be fired toward Israel in an act of desperation by Syria. Israel has indicated it would act in the face of such threats.
Clinton said that while the actions of 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.
Asked Monday if they were still considered secure, Pentagon press secretary George Little declined to comment about any intelligence related to the weapons.
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.
Fox News' Justin Fishel and The Associated Press contributed to this report.