ISTANBUL, Turkey - A leading Syrian opposition human-rights group Wednesday charged that the U.S.-led coalition was responsible for more than 100 civilian deaths since it began bombing Islamic State targets in September and it demanded that the U.S. Central Command carry out "a serious investigation" and stop issuing denials.

More than half those killed, 51, died Dec. 28, when U.S. aircraft struck a building housing an Islamic State prison in the northern Syria town of Al Bab, the Syrian Network for Human Rights said.

In addition, the group said that 29 civilians have died in bombings of oil refineries, many of them primitive operations run by local families to eke out a living in a war zone.

The network's report said the total number of confirmed civilian deaths since the U.S. began bombing Syria on Sept. 23 was 103, including 11 children and 11 women. Sixty-three deaths occurred since Dec. 14. The network's report included witness statements.

'Serious pursuit'

"Regrettably the alliance forces' Central Command denies that civilians have been killed by alliance forces," despite photographic and video evidence, the names of the victims and statements from victims' families, the group's director, Fadel Abdul Ghani, said in the report's introduction.

"There should be serious pursuit and investigation to hold the responsible accountable" and to compensate families so as to distinguish the U.S.-led alliance from "the lines of totalitarian dictatorships."

The bombing of the Saraya government building in Al Bab, an Islamic State headquarters that also housed a prison for local civilians, occurred Dec. 28. The U.S. Central Command didn't confirm that until two weeks later in response to queries by McClatchy.

'Not credible'

After McClatchy reported Jan. 11 that at least 50 civilians had died in the incident, Centcom said a review determined that the allegations of civilian casualties "are not credible." But it said it would look into the allegations if it were presented with substantive information.

Centcom used similar language Wednesday. "If there is new, substantive information provided to us regarding Al Bab, we welcome it and will certainly review," Col. Patrick S. Ryder, the Centcom spokesman, said in an e-mail to McClatchy.

Most of the prisoners were being held for three or four days for petty infractions of the Islamic State's draconian penal code.

Ryder said the coalition's targeting of income from oil production is fundamental to its effort to "degrade and ultimately defeat" the Islamic State.

But he said there were "significant mitigation measures" to reduce the number of civilian casualties and collateral damage.