U.S. Defense Secretary Leon Panetta told reporters Tuesday, however, that intelligence about new attempts by Syrian government forces to move chemical weapons "has really kind of leveled off."

U.S. officials said last week that they had seen intelligence suggesting that Syrian military units might be preparing chemical weapons for use, prompting strong warnings from international figures.

"We haven't seen anything new indicating any aggressive steps to move forward in that way," Panetta said.

"But we continue to monitor it very closely, and we continue to make clear to them that they should not under any means make use of these chemical weapons against their own population. That would produce serious consequences."

Panetta said he would like to believe that al-Assad has gotten the message: "We've made it pretty clear and others have as well."

But, he added, "You know it's also clear that the opposition continues to make gains in Syria, and our concern is that if they feel like the regime is threatened with collapse that they might resort to these kinds of weapons."

President Obama has said that any use of chemical weapons would cross a "red line," eliciting a swift U.S. reaction.

Syrian state-run media said Monday that the United States has falsely accused Syria of considering the use of chemical weapons.