The U.N. human rights chief has told the 47-nation Human Rights Council that there is little doubt that chemical weapons were used in Syria, but she did not say which of the combatants was suspected of using them.

U.N. High Commissioner for Human Rights Navi Pillay opened the council’s month-long session telling diplomats that “all the circumstances and responsibilities remain to be clarified.” She added that the use of chemical weapons is “one of the gravest crimes that can be committed.”

The Geneva-based council is due to receive an update Wednesday from its panel probing for war crimes and other human rights abuses in Syria, including the use of chemical weapons.

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