Intense fighting has left Syria's partial ceasefire in tatters. The truce was brokered by the United States and Russia to pave the way for the first peace talks attended by rebel factions since the crisis began five years ago.
As fighting raged and air strikes on rebel-held areas intensified, the opposition urged foreign states to supply them with the means to defend themselves, a thinly veiled reference to the anti-aircraft weapons long sought by insurgents.
Air strikes killed about 40 people in a crowded market on Tuesday in what may have been the worst incident of its kind since the cessation of hostilities took effect in February.
More than 40,000 people in camps, residential areas and settlements have been displaced owing to fighting in recent days, most pushed eastwards towards the strategically vital border town of Azaz, as well as the Bab al-Salam and Sijjou camps for internally displaced people, the United Nations said.