Thousands of people have been displaced and several hundreds are dead due to the airstrikes in Syria.Reuters

At least 12 aid workers and many civilians were feared killed after a United Nations convoy carrying food and supplies to Urm al-Kubra, a town to the west of Aleppo, Syria, was attacked through air strikes even as the threadbare ceasefire brokered by the United States and Russia fell through.

The US blamed Russia for the air strike, saying that the Vladimir Putin-led country was responsible for making sure the Assad regime did not take such steps. As soon as the temporary peace collapsed, the Syrian government conducted air raids at rebel-held areas.

The UN special envoy for Syria, Staffan de Mistura, called the incident an "outrage".

"The convoy was the outcome of a long process of permission and preparations to assist isolated civilians," he said in a statement emailed to Reuters.

At least 18 of 31 trucks were hit. The trucks belonged to Syrian Red Crescent and were carrying UN-supplied food for thousands of people. Apart from the aid workers, many civilians were also killed and injured in the attack.

"Let me be clear: if this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," Stephen O'Brien, the UN's emergency relief coordinator, said. "I call for an immediate, impartial and independent investigation into this deadly incident. The perpetrators should know that they will one day be held accountable for violations of international humanitarian and human rights law."

He added that the route for the convoy was provided to all parties involved in the conflict.

Five missiles attacked the trucks parked at a warehouse belonging to the Syrian Red Crescent in Urm al-Kubra, a witness was quoted as saying by Reuters.

At least 78,000 people are believed to be in need of immediate aid in the surrounding area.

We are deeply shocked that humanitarian workers and missions have yet again suffered from the brutality of this conflict," Ingy Sedky, of the International Committee of the Red Cross told AFP.