The United States has intelligence which raises "serious concerns" that Syria is considering employing chemical weapons in its war with rebel forces, US Defense Secretary Leon Panetta said Thursday.

"Without commenting on the specific intelligence that we have with regards to these chemical weapons, I think there is no question that we remain very concerned, very concerned, that, as the opposition advances, in particular in Damascus, that the regime might very well consider the use of chemical weapons," Panetta told reporters.

"The intelligence that we have raises serious concerns that this is being considered," he said.

US officials said privately this week that the Syrian regime has begun mixing precursor chemicals that could be used for sarin, a lethal nerve agent, and some media reports said the substance was loaded into bombs for warplanes.

Panetta repeated a US warning to Syrian President Bashar al-Assad not to resort to chemical weapons.

"The whole world is watching. The whole world's watching very closely, and the president of the United States has made very clear that there will be consequences, there will be consequences if the Assad regime makes a terrible mistake by using these chemical weapons on their own people," he said.

The comments came amid fierce fighting on the outskirts of Damascus and growing fears in Western capitals that the increasingly desperate regime might decide to use some of its chemical arms.

Washington worries that battlefield advances by rebels could prompt Assad to use chemical weapons, or that such stocks could become insecure or find their way into the hands of groups hostile to the United States and its allies.

US defense officials acknowledge commanders have prepared contingency plans and are consulting with allies in case US President Barack Obama opts for military action to secure chemical weapons sites.

Military options include bombing raids and deploying special forces but former intelligence officers and analysts say such a mission carries high risks with no guarantee of success.