"The (UN) Security Council should move swiftly to ensure accountability by imposing sanctions on individuals and entities responsible for chemical attacks in Syria," the New York-based rights watchdog said in a statement.
The April 4 attack in which sarin gas projectiles were fired into Khan Sheikhun, a rebel-held town in Idlib province in northwestern Syria, killed 83 people, according to the United Nations.
The Syrian Observatory for Human Rights gave a death toll of 87, including more than 30 children.
Ole Solvang, deputy emergencies director at HRW, said the panel's report "should end the deception and false theories that have been spread by the Syrian government".
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"Syria's repeated use of chemical weapons poses a serious threat to the international ban against the use of chemical weapons," Solvang said.
"All countries have an interest in sending a strong signal that these atrocities will not be tolerated."