Syria has started mixing chemicals that will be able to be used to make the deadly sarin gas, AFP reported citing a U.S. official.

AFP quoted an anonymous official as saying the U.S. has "picked up several indications which lead us to believe that they're combining chemical precursors."

The official reportedly said the operation was apparently aimed at making sarin.

U.S. Secretary of State Hillary Clinton earlier warned that Washington will take action if Syria uses chemical weapons.

"I am not going to telegraph any specifics what we do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people, but suffice to say, we are certainly planning to take action if that eventuality were to occur," she said during a visit to Prague.