The Syrian Observatory for Human Rights said the dead included women and children and the death toll from the air strikes, which hit a camp for internally displaced people near the town of Sarmada, was likely to rise.
Footage shared on social media showed rescue workers putting out fires which still burned among charred tent frames, pitched in a muddy field. White smoke billowed from smouldering ashes, and a burned and bloodied torso could be seen in the footage.
"There were two aerial strikes that hit this makeshift camp for refugees who have taken refuge from fighting in southern Aleppo and Palmyra," said Abu Ibrahim al-Sarmadi, an activist from the nearby town of Atmeh who spoke to people near the camp.
Nidal Abdul Qader, an opposition civilian aid official who lives about 1 km (half a mile) from the camp, said around 50 tents and a school had burned down.
United Nations aid chief Stephen O’Brien said he was horrified and sickened by what had happened and called for an investigation.
Toner sought to address confusion over the timeline for the cessation of hostilities, with Syrian state media saying the army would abide by a "regime of calm" in Aleppo for 48 hours and the State Department emphasising it was open-ended.
Russia blocked a British-drafted UN Security Council statement, which would have condemned the surge in violence in Aleppo and attacks against civilians.
"There is one country that could not agree it and it's Russia," Britain's UN Ambassador Matthew Rycroft told reporters. "That does speak volumes about their support for protection of the Assad regime."
Gas field captured
The army blamed Islamist insurgents for violating the agreement overnight, saying they had shelled government-held residential areas of Aleppo indiscriminately. Residents said the violence eased by morning and more shops opened up.
Heavy fighting was reported in the southern Aleppo countryside near the town of Khan Touman, where al Qaeda's Syrian branch Nusra Front is dug in close to a stronghold of Iranian-backed militias, a rebel source said.
Rebels were attacking government positions around the town and government forces carried out air attacks on the area, pro-Syrian government television channel Al-Mayadeen and the Observatory said.
In the east of the country, Islamic State militants captured the Shaer gas field, their first gain in the Palmyra desert area since they lost the ancient city in March, according to rebel sources and a monitor.