At least five people, including three children, were killed on Tuesday in a Syrian regime attack on a school in the Damascus suburb of Eastern Ghouta.

Eastern Ghouta is located inside one of the recently established de-escalation zones, where attacks are forbidden.

“Mortar shells struck a school in Jisreen town, killing five and injuring 15 others,” Mahmoud Ethem, a member of the Syrian Civil Defense, also known as White Helmets, told Anadolu Agency.

The main opposition stronghold on the outskirts of Damascus, Eastern Ghouta has remained under siege by Syrian regime forces since December 2012.

In May, Russia, Turkey, and Iran signed a deal in the Kazakh capital Astana to establish de-escalation zones in war-torn Syria to ensure the continuation of a cease-fire deal reached last December.

During the talks, Eastern Ghouta was designated part of a network of de-escalation zones in which acts of aggression would be expressly forbidden.

Russia and Iran are main backers of the Bashar al-Assad regime, while Turkey supports the Syrian opposition.

Syria has been locked in a vicious civil war since early 2011 when the Assad regime cracked down on pro-democracy protests with unexpected ferocity. Since then, hundreds of thousands of people have been killed and more than 10 million displaced, according to UN officials.