Days after the Pentagon announced that it would investigate alleged instances of civilian deaths in airstrikes carried out by the U.S.-led coalition in its campaign against the Islamic State group, McClatchyDC News claimed, in a report, that at least 50 civilians were killed in an airstrike in the northern Syrian town of Al Bab in December last year. If confirmed, this would make it the single worst case of civilian casualties since the airstrikes in Syria began in September.

The report, published Sunday, cited eyewitnesses and officials from the Syrian Network for Human Rights (SNHR), a UK-based human rights group. Although the U.S. Central Command (CENTCOM) confirmed that an airstrike targeting “an ISIL headquarters building” in Al Bab had been carried out on Dec. 28, it reportedly said that there was no evidence of civilians being killed in the incident.

However, officials from SNHR reportedly told McClatchy that at least 80 people, including 25 ISIS militants and 55 civilians, were killed in the airstrike. Witnessed interviewed by McClatchy also said that the airstrike, which targeted a building that had been turned into a makeshift prison by the Islamic State group, had been “leveled to the ground.”

“The missile was very powerful and destroyed the building completely,” Huda al Ali, a spokesperson for SNHR, reportedly said.

The report comes less than a week after the U.S. Department of Defense acknowledged, for the first time since the start of the U.S.-led campaign against the Islamic State group, that airstrikes in Iraq and Syria could have led to civilian deaths. Although officials from CENTCOM had announced that they were investigating two instances of civilian deaths -- one in Iraq and one in Syria -- it has not been disclosed whether the airstrike at Al Bab is also being investigated.

The U.S.-led coalition has carried out over 1,400 airstrikes on ISIS strongholds in Iraq and Syria since the aerial offensive began last August, according to media reports. In its latest report, released Sunday, CENTCOM said the U.S. and coalition forces had carried out 19 airstrikes on ISIS strongholds in Iraq and Syria over the weekend.