President Barack Obama, in a speech at the National Defense University on Monday, pointedly warned Syrian President Bashar Assad not to use the weapons.
"Today I want to make it absolutely clear to Assad and those under his command: The world is watching," Obama said. "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."
Secretary of State Hillary Rodham Clinton, in Prague for meetings with Czech officials, said she wouldn't outline any specifics.
"But suffice it to say, we are certainly planning to take action if that eventuality were to occur," Clinton said.
Options now being considered range from aerial strikes to limited raids by regional forces to secure the stockpiles, according to one current U.S. official, and one former U.S. official, briefed on the matter. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.
The administration remains reluctant to dispatch U.S. forces into Syria, but a U.S. special operations training team is in neighboring Jordan, teaching troops there how to safely secure such sites together with other troops from the region, the officials said.
The warnings come after U.S. intelligence detected signs the Syrian regime was moving the chemical weapons components around within several of Syria's chemical weapons sites in recent days, according to a senior U.S. defense official and two U.S. officials speaking on Monday. Two officials said the activities did not involve movement of components in or out of various sites, but the movement was confined to within the individual locations.
One of the officials said they were seeing activities they had not seen before that bear further scrutiny.
The officials spoke on condition of anonymity because they were not authorized to speak publicly about intelligence matters.
According to another senior US official, the US is worried about "indications of preparations" for a possible use of the chemical weapons. The U.S. still doesn't know whether the regime is planning to use them, but the official says there is greater concern because there is the sense that the Assad regime is under greater pressure now.
U.S. intelligence officials also intercepted one communication within the last six months they believe was between Iran's infamous Quds Force, urging Syrian regime members to use its supplies of toxic Sarin gas against rebels and the civilians supporting them in the besieged city of Homs, the former U.S. official said. That report was not matched by other intelligence agencies, and other intelligence officials have said Iran also does not want the Syrians to use their chemical weapons.
The Assad regime insists it would not use such weapons against Syrians, though it carefully does not admit to having them. The Ministry of Foreign Affairs said the government "would not use chemical weapons - if there are any - against its own people under any circumstances." The regime is party to the 1925 Geneva Protocol banning chemical weapons in war.
The Syrian assurances did not placate the White House.
"We are concerned that in 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," said White House press secretary Jay Carney.
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 them, or the intent to 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 this person was not authorized to speak publicly about the issue.
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.
Syria has some 75 sites where weapons are stored, but U.S. officials aren't sure they have tracked down all the locations, and fear some stockpiles may have already been moved. Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads, plus several tons of material stored in either large drums, or in artillery shells, which become deadly once fired.
"In Syria, they have everything from mustard agent, Sarin nerve gas, and some variant of the nerve agent VX," according to James Quinlivan, a Rand Corp. analyst who specializes in the elimination of weapons of mass destruction.
A primary argument against sending in U.S. ground troops is that whoever takes possession of the chemical weapons will be responsible for destroying them, as part of the 1997 Chemical Weapons Convention. Destroying Syria's stockpiles could cost hundreds of millions of dollars, and take more than a decade, Quinlivan said.
Syria's 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.
Activity has been detected at Syrian weapons sites before.
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.
Senior lawmakers were notified last week that U.S. intelligence agencies had detected activity related to Syria's chemical and biological weapons, said a U.S. intelligence official, speaking on condition of anonymity to discuss the closed-door meetings. All congressional committees with an interest in Syria, from the intelligence to the armed services committees, are now being kept informed.
"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.
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.
Associated Press writers Bradley Klapper in Prague, Josef Federman in Jerusalem, Albert Aji in Damascus and Matthew Lee, Lolita C. Baldor and Julie Pace in Washington contributed to this report.