chemical weapons
According to Human Rights Watch, the attacks were part of the overall military strategy to retake the city.
The probe shows the decision to use toxic weapons came from the very top.
The threat from chemical attacks may be limited, but the fear they create is real.
Up to 250 people may have died from exposure to chemical weapons, Amnesty International says.
WHAT'S HAPPENING
No troops were injured in the shelling, officials told CNN.
Given their enormous blast, fire, and radioactivity, their explosion would bring an end to virtually all life on earth.
The report sets up a UN Security Council showdown between members on how to respond.
Syria agreed in September 2013 to destroy its entire chemical weapons program under a deal negotiated with the United States and Russia.
The militants allegedly attacked Kurdish forces and civilians with poison gas.