At least 16 people, among them 13 children, were killed in Syrian government airstrikes on a town in the southern Daraa province on Tuesday, a monitor said, according to AFP.

The Syrian Observatory for Human Rights said the strikes hit a center for study of the Koran in the town of Eastern Ghariyah.

The Britain-based monitor accused Syria’s government of a “massacre,” saying a woman was also among those killed in the strikes.

The group said the toll in the strikes, which hit on Tuesday afternoon, was expected to rise further because a number of those injured were in serious condition.

Eastern Ghariyah lies near the eastern edge of Daraa province, not far from the 52nd Brigade base that a rebel alliance seized from the regime a week ago.

The Southern Front alliance seized the base in a surprise attack, before pushing across the provincial border into Sweida and briefly overrunning the Al-Thalaa air base, according to AFP.

Regime forces expelled the rebels from Al-Thalaa on Friday, but fighting has continued in the area.

Rights groups have regularly criticized Syria’s government for indiscriminate fire on civilian areas and specifically for the use of “barrel bombs” – crude weapons, made of containers packed with explosives and scrap metal, that are typically dropped from helicopters.

On May 31, barrel bombs dropped on rebel-controlled areas throughout Aleppo province killed more than 80 people, one of the highest daily tolls since the beginning of 2015. UN’s Syria peace envoy Staffan de Mistura later called the civilian deaths “totally unacceptable”.

Assad, however, has repeatedly denied using barrel bombs and has suggested in interviews that no such weaponry exists.