Syrian pilots used sarin gas against civilians in the opposition-held area of Khan Shaykhun, Idlib province on April 4, killing more than 80 civilians, U.N. war crimes commissioners said on Wednesday.
The U.N. Commission of Inquiry on Syria in a report released in Geneva said most of the dead in Khan Shaykhun were women and children.
This attack was part of a Syrian regime "pattern of using chemical weapons against civilians in opposition-held areas," UN investigators said.
Noting concerns about the impact of international coalition air strikes on civilians, the report highlighted U.S. operations which hit civilians.
The report said the U.S. broke international law when attacking a mosque in Aleppo on March 16.
"In Al-Jinah, Aleppo, forces of the United States of America failed to take all feasible precautions to protect civilians and civilian objects when attacking a mosque, in violation of international humanitarian law," the report said.
The report also said the ongoing offensive to repel Daesh has displaced more than 190,000 people in Raqqa.
"The coalition air strikes have reportedly resulted in significant numbers of civilians killed and injured. Investigations are ongoing," it added.