Handout photo provided to Reuters on February 13, 2017, by Human Rights Watch claiming to show remnant of a yellow gas cylinder found in Masaken Hanano, Aleppo, after a chlorine attack on November 18, 2016. Courtesy of Human Rights Watch/Handout via A still image from an undated video provided to Reuters on February 13, 2017, by Human Rights Watch claiming to show people treated in Aleppo, Syria, following a gas attack.