At least 18 trucks in a 31-vehicle convoy delivering aid in Syria were hit on Monday in an attack a monitoring group said left 12 dead, prompting outrage from the United Nations.
The convoy from the UN and the Syrian Arab Red Crescent (SARC) was en route to the hard-to-reach town of Orum al-Kubra, in Aleppo province, to deliver humanitarian assistance to 78,000 people, the UN said.
The Syrian Observatory for Human Rights said 12 aid workers and drivers were killed in the strike, but the United Nations did not confirm the casualties.
"Our outrage at this attack is enormous," the UN envoy for Syria, Staffan de Mistura, told reporters.
"The convoy was the outcome of a long process of permission and preparations to assist isolated civilians."
Jan Egeland, head of the UN humanitarian task force for Syria, tweeted that there were "many killed and injured" in the strike.
The convoy was "bombed today in spite of de-confliction with parties" in which the aid agencies coordinate their movements with all sides on the ground, he added.
UN aid chief Stephen O'Brien said he was "deeply concerned" by the incident and called on "all parties to the conflict, once again, to take all necessary measures to protect humanitarian actors, civilians, and civilian infrastructure as required by international humanitarian law."
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A SARC warehouse was also hit, UN spokesman Stephane Dujarric said.
"Multiple sources have confirmed that the town was shelled this evening," he said.
"However, the situation and reporting on it continues to evolve and we are unable at this time to independently verify casualties," he added.
The Syrian observatory said President Bashar al-Assad's air force carried out more than 40 air raids since the army announced the end of the ceasefire, killing 36 people.
French Foreign Minister Jean-Marc Ayrault strongly condemned the strike, saying the destruction of the aid convoy underscored "the urgency of a cessation of hostilities in Syria."
The International Committee of the Red Cross (ICRC) confirmed the attack on the convoy.
"The situation on the spot is very chaotic and we are deeply shocked that humanitarian workers and missions have yet again suffered from the brutality of this conflict," ICRC spokeswoman Ingy Sedky told AFP.
Syria's military earlier announced the end of a ceasefire brokered by the United States and Russia, accusing rebels of more than 300 violations and failing to "commit to a single element" of the US-Russia deal.