Iran has been accused of helping train and assist Syrian scientist with the construction of chemical plants, it said.

The United States is paying particular attention to the possibility of the weapons falling into the hands of extremists in the event the government loses control of certain areas or splinters within itself, the defense official said. U.S. intelligence and military officials have raised the concern that al Qaeda in Iraq and other extremists have infiltrated the opposition groups.

"Several dangerous terrorist groups, including al Qaeda in Iraq, are active in Syria, and I am very concerned that as the situation in Syria deteriorates, these weapons could fall into the wrong hands," said Rep. Mike Rogers, R-Michigan, chairman of the House Permanent Select Committee on Intelligence.

"Loose chemical weapons in Syria are exactly the type of opportunity AQ has been looking for," he said. "We cannot discount that the Assad regime could make a decision to use these weapons in an act of desperation, and we must act accordingly."