The United Nations has said raids and air strikes by the Syrian government and its allies on an area “packed with civilians” most likely violates international law.
Earlier, a deal to suspend fighting and evacuate rebel fighters and civilians from eastern Aleppo had stalled, with resumed heavy shelling reported.
Syrian activists say air strikes resumed over rebel-held territory, where at least 50,000 civilians remain.
The renewed violence came despite a ceasefire put in place less than a day earlier.
Zeid Raad al-Hussein, the UN High Commissioner for Human Rights, said the Syrian government had “a clear responsibility to ensure its people are safe”.
In a statement, he said: “The way this deal was dangled in front of this battered and beleaguered population – causing them to hope they might indeed live to see another day – and then snatched away just half a day later is also outrageously cruel.”
Meanwhile, the BBC has learned that Western forces are using satellites and unmanned aircraft to gather evidence of possible war crimes in Aleppo and elsewhere in Syria.