The US says it is “appalled” by reports that Syrian government air strikes killed more than 30 people in a rebel-held suburb of Damascus on Thursday.

The Syrian Observatory for Human Rights said 12 children were among those who died in the raids near a school and a hospital in Deir al-Asafir. The US said it condemned any attacks directed at civilians.

The air strikes came despite a month-long cessation of hostilities between government and rebel forces. The partial truce has resulted in a significant reduction in violence in much of Syria and has largely held despite both sides accusing each other of violations.

Thursday’s attack on Deir al-Asafir, a town in the rebel-dominated rural eastern Ghouta region, was one of the deadliest incidents involving civilians since the cessation of hostilities began on 27 February.

The Syrian Observatory, a UK-based monitoring group, said that 12 children, nine women, a teacher and a volunteer emergency response worker from the Syrian Civil Defence, an organisation also known as the “White Helmets”, were among the 33 people killed. The Syrian Civil Defence published a video of what it described as the last moments of the emergency worker, Mohammed Walid al-Ghorani. It showed an ambulance arriving at the scene of one of the strikes in Deir al-Asafir and a man, carrying a stretcher over his shoulder, rushing towards damaged buildings.

The organisation also published photos showing the destruction of its headquarters in the town, including a fuel depot. Fire hoses, extinguishers and helmets could be seen among the rubble.

The US – which backs the opposition to President Bashar al-Assad and is monitoring the cessation of hostilities in co-ordination with his staunch ally, Russia – said late on Thursday that it condemned “in the strongest terms any such attacks directed at civilians”. “In joining the cessation of hostilities, even apart from its commitments to avoid attacking groups participating in the cessation of hostilities, the regime committed to full implementation of UN Security Council Resolution 2254, which called for an immediate end to any attacks against civilians and for all parties to comply with their obligations under international law,” state department spokesman John Kirby said in a statement.

Much of the eastern Ghouta is in the hands of various armed factions that are covered by the cessation of hostilities. However, the al-Qaeda-linked al-Nusra Front also has a presence in the area and it is not included, along with the rival jihadist group Islamic State (IS).

The Syrian Observatory said government troops had been trying to encircle Deir al-Asafir, which is home to about 2,700 families, for several weeks.