The Syrian government has moved some of its chemical weapons to safety as it battles rebel forces, US Defence Secretary Leon Panetta says.
He said there was intelligence that there had been "limited movement" to secure the chemicals, but that "the major sites still remain in place".
Syria has admitted to having a large stockpile of chemical weapons.
US President Barack Obama has warned Damascus it would be held accountable if it uses them.
Mr Panetta told a news conference at the Pentagon on Friday: "We continue to have a concern about the security of the CBW (chemical and biological weapons) sites."
But he said the major sites "still remain secured by the Syrian military".
Mr Panetta added: "There has been some intelligence that with regards to some of these sites that there has been some movement in order for the Syrians to better secure... the chemicals."
Syria, which has not signed the Chemical Weapons Convention, is believed to possess mustard gas and sarin, a highly toxic nerve agent.
The CIA has said those weapons "can be delivered by aircraft, ballistic missile, and artillery rockets".
However there is no evidence that it has used them in the 18-month conflict with rebel forces.
In the latest fighting, fierce fighting has been reported in the second city, Aleppo. The rebel Free Syrian Army (FSA) said on Thursday that a "decisive" battle had begun in the city.
Residents and activists described the clashes as "unprecedented", saying rebel fighters were attacking government positions on several fronts.
Syrian state media reported some unusual mortar fire onto south-eastern parts of Aleppo that have so far been relatively untouched.
Attacks by rebels had been repulsed in several places, and heavy losses had been inflicted by government forces, they said.
For more than a month, the struggle for control of Aleppo has been at a stalemate, with government forces unable to dislodge the rebels, and the latter unable to take complete control, the BBC's Jim Muir in neighbouring Lebanon says.
Meanwhile, several north-eastern and southern districts of Damascus which were supposedly recaptured by the army weeks ago also saw further violence on Friday, with troops and militia moving back in.