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, reports CNN.

Officials from the UN and US said they were "disgusted" and "outraged" by the incident, which according to the UN saw 18 of 31 trucks in the aid convoy hit.

Twelve people involved in the aid delivery were killed, according to the Syrian Observatory for Human Rights, a UK-based organization that monitors the conflict in Syria.

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

The Syrian Civil Defense, a volunteer EMS service, posted video of the aftermath of the attack on social media. CNN cannot confirm its authenticity nor claims made in the video that the Syrian regime was responsible.

A Syrian Arab Red Crescent warehouse was also hit, according to the UN. The aid convoy and the warehouse were both in the area of Urum al-Kubra, a reportedly rebel-held town west of Aleppo.

The SOHR said at least 32 people in total were killed in separate attacks in Aleppo and its western suburbs on Monday. The violence came just hours after Syrian authorities declared that a fragile ceasefire in the war-torn country was over.

Getting aid to areas cut off by fighting has been a growing concern for humanitarian agencies - with trucks destined for eastern Aleppo, where an estimated 250,000 civilians have been short of food, medicine and water, prevented from getting through.

"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."

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.

The State Department also said it was outraged by the reports: "The destination of this convoy was known to the Syrian regime and the Russian federation and yet these aid workers were killed in their attempt to provide relief to the Syrian people."

Before news of the attack broke, neither the US nor Russia - the countries which brokered the pact - publicly said that the ceasefire is over.