The U.S. government is concerned that Syrian President Bashar al-Assad may use chemical weapons against his own people to end a revolt that has now dragged on for 21 months and killed at least 40,000 people.

Jay Carney, White House spokesman, said such a measure would “cross a red line,” suggesting foreign military intervention in Syria. He also noted that U.S. and allied officials are closely monitoring Syria’s chemical weapons facilities.

Carney added “the world is watching” Assad and that he would be held accountable for his actions.

Separately, the United Nations said it is withdrawing "all non-essential international staff" from Syria.