The United Nations expressed outrage on Monday after an aid convoy was hit in Syria and warned that if the attack was deliberate, it would amount to a war crime.
At least 18 trucks in the 31-vehicle convoy were struck in an attack a monitoring group said left 12 aid workers and drivers dead.
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.
"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.
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.
The United Nations did not confirm the casualties cited by the Syrian Observatory for Human Rights.
O'Brien said initial reports indicate that "many people" were killed or seriously injured, including SARC volunteers.
A SARC warehouse was also hit and a health clinic was reportedly damaged, O'Brien said.
French Foreign Minister Jean-Marc Ayrault strongly condemned the strike, saying it underscored "the urgency of a cessation of hostilities in Syria."
The United States said it was outraged and stressed that the destination of the convoy was known to the Syrian regime and its ally, Russia.
"Notification of the convoy -- which planned to reach some 78,000 people -- had been provided to all parties to the conflict and the convoy was clearly marked as humanitarian," O'Brien said.
"There can be no explanation or excuse, no reason or rationale for waging war on brave and selfless humanitarian workers trying to reach their fellow citizens in desperate need of assistance."
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.