Washington says more than 1,400 people, many of them children, were killed in the world's worst use of chemical arms since Iraq's Saddam Hussein gassed thousands of Kurds in 1988. 

U.S. military action will be put to a vote in Congress, which ends its summer recess on September 9, giving President Bashar al-Assad time to prepare the ground for any assault and try to rally international support against the use of force. 

In a letter to U.N. chief Ban Ki-moon and President of the Security Council Maria Cristina Perceval, Syrian U.N. envoy Ambassador Bashar Ja'afari called on "the U.N. Secretary General to shoulder his responsibilities for preventing any aggression on Syria and pushing forward reaching a political solution to the crisis in Syria", state news agency SANA said on Monday.