A suspected chemical attack killed dozens of people in a rebel-held area of Syria on Tuesday, activists said, describing an incident that would mark one of the worst of its kind in Syria’s six-year civil war.

At least 58 people, including 11 children, died in the incident in northern Idlib province, southwest of Aleppo, said the British-based Syrian Observatory for Human Rights. The death toll “is expected to rise” because more than 160 people were injured, many experiencing severe symptoms, the group added.

White House Press Secretary Sean Spicer told reporters that the chemical attack is “reprehensible and cannot be ignored by the civilized world.”