Barack Obama has warned Syrian President Bashar Assad of the "consequences" if he were to make the "tragic mistake" of using chemical weapons against his own people.
The US President's remarks came as security officials said they had detected activity at chemical weapons depots.
"I want to make it absolutely clear to Assad and those under his command, the world is watching. The use of chemical weapons is and would be totally unacceptable," he said.
Secretary of State Hillary Clinton echoed Mr Obama's caution that such action would cross a "red line" for the United States that would prompt action.
She did not, however, address the reports of movement at Syrian chemical weapons depots - or go into detail about what the response might be - but insisted that Washington would address any threat that arises.
"We have made our views very clear: This is a red line for the United States," she said on Monday while in Prague for meetings with Czech officials.
"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 added: "We once again issue a very strong warning to the Assad regime that their behaviour is reprehensible, their actions against their own people have been tragic.
"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 utilising their chemical weapons."
White House press secretary Jay Carney added: "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."
The warning came as Lebanese security officials said Syria's foreign ministry spokesman had left the country and was heading to the UK on a flight from Beirut.
However, it was unclear whether Jihad Makdissi had defected from Mr Assad's regime.
Syria has been careful never to confirm that it has any chemical weapons - any use of which would draw international condemnation.
Its ministry of foreign affairs said Syria "would not use chemical weapons - if there are any - against its own people under any circumstances".
Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads, and there are concerns its chemical and biological weapons could fall into the wrong hands should the regime in Syria collapse and lose control of them.
The US has opposed military intervention or providing arms support to Syria's rebels for fear of further escalating a conflict that activists say has killed more than 40,000 people since March 2011.
Meanwhile, the United Nations said it was suspending its aid operations in Syria and withdrawing all non-essential international staff due to attacks on humanitarian aid convoys and the hijacking of goods or vehicles in recent weeks.
The European Union said it too was scaling back its activities in the Syrian capital Damascus due to the worsening security situation.