WASHINGTON - Eleven civilians died in coalition air strikes against the Islamic State group in Iraq and Syria late last year, officials said Thursday.

The acknowledgement brings to 199 the total minimum number of unintentional deaths from the campaign that started in the fall of 2014, though critics say the real number is far higher.

A statement from the coalition said investigators had probed a series of reports alleging civilian deaths from air strikes.

Seven of these reports were deemed non-credible, while four others, dating from October to December, were credible.

In one incident, seven civilians were killed in a December 7 strike on an IS compound near the group's Syrian stronghold Raqa.

"Although the coalition makes extraordinary efforts to strike military targets in a manner that minimizes the risk of civilian casualties, in some cases casualties are unavoidable," the coalition statement read.

A December 9 strike saw two civilians killed near Mosul in Iraq.

Though the coalition did not provide details, unintentional deaths sometimes occur when a civilian enters a bomb's zone of destruction after that munition has been released from a plane or drone circling high overhead.

It can take about 30 seconds for a bomb to reach its target.

Airwars, a London-based collective of journalists and researchers, uses local sources, photographs and media accounts to keep a detailed list of every known coalition air strike.

They have praised Pentagon efforts at accountability compared to other players in Syria such as Russia and the regime of President Bashar al-Assad, but the group says the number of likely civilian deaths from coalition strikes is 2,358 at a bare minimum.