Human Rights Watch said in a report that these attacks "are part of a broader pattern of Syrian government forces' use of chemical weapons" which could be categorized as crimes against humanity.
The rights group said the April attack in Khan Sheikhoun and the three others using suspected nerve agents all took place in areas where offensives by armed forces fighting the government threatened military air bases.
In the December attacks, Human Rights Watch said four witnesses interviewed by phone and two medical personnel interviewed by text messages through intermediaries gave consistent accounts. An opposition-affiliated activist and local residents provided the names of 64 people who died of chemical exposure, it said.
The group quoted residents, medical personnel and first responders saying the suspected attack on March 30 caused no deaths but injured dozens of people.
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As part of the evidence showing that attacks have become widespread and systematic, Human Rights Watch identified three different systems being used to deliver chemical weapons: Government warplanes dropping bombs with nerve agents.
Government helicopters dropping chlorine-filled munitions, which has become more systematic.
Government or pro-government ground forces starting to use improvised ground-launched munitions filled with chlorine.
In at least some attacks, the rights group said, the aim appears to have been to inflict "severe suffering" on the civilian population.
"In the last six months, the government has used warplanes, helicopters and ground forces to deliver chlorine and sarin in Damascus, Hama, Idlib and Aleppo," he said. "That's widespread and systematic use of chemical weapons.
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