WASHINGTON: The Syrian military has loaded precursor chemicals for the deadly nerve gas sarin into aerial bombs and is awaiting final orders from President Bashar al-Assad, NBC News reported Wednesday.

If confirmed, the move would mark a step further in Syria's progression toward possibly using chemical weapons.

US officials told NBC News that the loaded aerial bombs could be dropped onto the Syrian people from dozens of fighter jets.

But they stressed that the sarin bombs had not yet been loaded onto planes and that Assad had not yet issued a final order to deploy them. However, if he goes ahead, "there's little the outside world can do to stop it," one official said.

CNN reported that the Israeli, Jordanian, Lebanese and Turkish intelligence services were in close contact with their US counterparts to decide on the next steps.

The Syrian government, fighting to prevent the capital Damascus from falling to rebel forces, has insisted it would never resort to chemical weapons.

But a US official told AFP on Tuesday that Syria had begun mixing chemicals that could be used to make sarin, while CNN reported Damascus could use the gas in a limited artillery attack on advancing rebels.

Washington fears that battlefield advances by rebels could prompt Assad to use chemical arms, 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 President Barack Obama on Monday warned Assad of "consequences" if he uses chemical weapons against his own people, in a new warning as the conflict approaches the 21-month mark with more than 41,000 people killed.