At least 12 aid workers killed in Syria airstrike on a convoy carrying aid to war-torn Aleppo after ceasefire ends

An aid convoy carrying life-saving aid to 78,000 people was attacked near the Syrian city of Aleppo Monday, according to the United Nations.

Air strikes hit around 20 aid trucks outside a Red Crescent center in the Syrian province of Aleppo hours after the Syrian army declared a week-long truce to be over.

The Britain-based Observatory for Human Rights reported on Monday that among the dead were at least 12 people who had been killed in an attack that reportedly hit a convoy of aid trucks. The monitoring group could not confirm the definite number of casualties.

At least 18 of 31 trucks in a U.N. and Syrian Arab Red Crescent convoy were hit along with a Syrian Red Crescent (SARC) warehouse, said U.N. spokesman Stephane Dujarric. The convoy was delivering aid for 78,000 people in the hard-to-reach town of Urm al-Kubra in Aleppo Governorate, he added.

No one has claimed responsibility for the attacks. It’s not clear whether the convoy was hit by an airstrike or shelled.

“Our outrage at this attack is enormous,” said Staffan de Mistura, the UN’s special representative for Syria. “The convoy was the outcome of long process of permission and preparations to assist isolated civilians.”

Photos of the attack were shared on social media soon after, showing destruction in the area:

Stephen O’Brien, the head of the UN’s relief organization, said he was “disgusted” by the reports and said if it’s discovered that aid workers were deliberately targeted, that the strike would amount to a war crime.

UN aid chief Stephen O’Brien called for an investigation. “Let me be clear: if this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime,” he said. A Syrian Arab Red Crescent (SARC) warehouse was also hit and a health clinic was reportedly damaged, O’Brien said.

More than 300,000 people have been killed and millions displaced since the Syria conflict started in 2011 with anti-government protests.