– A U.S.-led coalition airstrike killed at least 50 Syrian civilians in late December when it targeted a headquarters of Islamic State extremists in northern Syria, according to a witness and a Syrian opposition human rights organization.

The civilians were being held in a makeshift jail in the town of Al Bab, close to the Turkish border, when the aircraft struck on the evening of Dec. 28, witnesses said. The building, called the Al Saraya, a government center, was leveled in the airstrike. It was days before civil defense workers could dig out the bodies.

The U.S. Central Command, which had not announced the airstrike, confirmed the attack Saturday in response to repeated inquiries. “Coalition aircraft did strike and destroy an ISIL headquarters building in Al Bab on Dec. 28,” Col. Patrick Ryder wrote in an e-mail.

He said a review of the airstrike showed no evidence of civilian casualties but offered to examine any additional information, “since we take all allegations seriously.”

U.S. officials acknowledged for the first time last week that they are investigating “at least a few” claims of civilian casualties as a result of airstrikes on Syria. “This is something we always take seriously,” said Navy Rear Adm. John Kirby, a Pentagon spokesman. “We are very mindful of trying to mitigate the risk to civilians every time we operate, everywhere we operate.”

A subsequent e-mail from Central Command said the Pentagon had received nine reports of civilian deaths in Syria and that determinations were still to be made in four. No details were provided.

But the Syrian Network for Human Rights, an independent opposition group that tracks casualties in Syria, said it has documented the deaths of at least 40 civilians in airstrikes in the months between the start of U.S. bombing in Syria Sept. 23 through the Dec. 28 strike on Al Bab. The deaths include 13 people killed in Idlib Province on the first day of the strikes. Other deaths include 23 civilians killed in the eastern province of Deir el Zour, two in Raqqa Province and two more in Idlib Province.

The issue of civilian deaths in U.S. strikes is critical as the United States hopes to win support from Syrians for its campaign against the Islamic State in Iraq and the Levant. The deaths are seen by U.S.-allied moderate rebel commanders as one reason support for their movement has eroded in northern Syria while support for such radical forces as Al-Qaida’s Nusra Front and the ISIL has gained.

Rebel commanders say they have intelligence that could avoid civilian casualties, but that U.S. officials refuse to coordinate with them.

News of casualties from U.S. actions in Syria rarely seeps out from towns like Al Bab, which has a population of 150,000, because ISIL has been able to close it off by threatening to jail or kill those reporting to the outside world.

The Central Command, on behalf of the Joint Task Force, generally issues reports of airstrikes on the day they occur, but for a while was publishing its reports only three days a week. The Al Bab strike was not included in any of the summaries, however.

Central Command spokesman Ryder said the failure to list the Dec. 28 airstrike was an administrative oversight.

McClatchy reporters found two sources who confirmed a high civilian death toll from the airstrike. One witness, an activist in Al Bab, gave the death toll as 61 civilian prisoners and 13 ISIL guards. The Syrian Network for Human Rights estimated the death toll at 80, and said 25 of those were ISIL guards and another 55 were either civilians or imprisoned fighters from other rebel groups.

Either number would make the Al Bab strike the single worst case of civilian deaths since the U.S. began bombing targets in Syria.

The U.S.-led campaign against the militants, who have seized about a third of Syria and much of northern and western Iraq, is being planned by analysts hundreds, sometimes thousands, of miles from the battlefield.