The team from the U.N. and the chemical weapons watchdog blamed the government for using chlorine gas in two attacks and Islamic State fighters for using mustard gas in one attack. Calling the use of chemical weapons "a barbaric tool, repugnant to the conscience of mankind," U.S. Ambassador Samantha Power urged the Security Council to take "strong and swift action" against the perpetrators. In September 2013, Syria accepted a Russian proposal to relinquish its chemical weapons stockpile and join the Chemical Weapons Convention. The United States will seek accountability at the U.N.