Barack Obama, the US president, has warned Syrian President Bashar al-Assad not to use chemical weapons on his own people, saying there would be consequences if he were to do so.
"Today I want to make it absolutely clear to Assad and those under his command: The world is watching," he said on Monday.
"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."
Syrian state media quoted its foreign ministry as saying that it would "never under any circumstances" use such weapons, "if such weapons exist".
Hillary Clinton, the secretary of state, who is in Prague, the capital of Czech Republic, said she would not outline any specifics.
"But suffice it to say, we are certainly planning to take action if that eventuality were to occur," she said.
Earlier in the day, a senior White House spokesman said that the US and its allied intelligence had monitored Syrian movement of chemical weapons components in recent days.
"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," Jay Carney, press secretary, said at a White House briefing.
"And as the president has said, any use or proliferation of chemical weapons by the Syrian regime would cross a red line for the United States."
Al Jazeera's Alan Fisher, reporting from Washington, said: "It's clear there is a concern on the Americans' part, as there has been since the start of the rebel uprising in Syria, that perhaps the Assad regime could turn to its chemical weapons stockpile - something it says that it may not have - to attack its own people."