The UN has suspended all aid convoys in Syria after a devastating attack on its lorries near Aleppo on Monday.
The strikes, which witnesses say came from the air, came hours after Syria declared a week-old US-Russia brokered cessation of hostilities at an end.
Russia and Syria have both insisted that their forces were not involved.
But UN chief Ban Ki-moon launched a stinging attack on the Syrian government, saying it had killed the most civilians in the civil war.
In unusually blunt language, Mr Ban said "powerful patrons... feeding the war machine, also have blood on their hands".