Twenty-one civilians were killed in US-led coalition air strikes against Islamic State targets in Iraq and Syria in recent months, officials acknowledged today.

The deaths bring to at least 220 the number of civilians unintentionally killed since operations to defeat IS began in late summer 2014, though critics say the real number is a lot higher.

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

In a January 13 strike on IS fighters in a house, investigators determined that eight civilians were unintentionally killed.

"During post-strike video analysis, civilians were identified near the house who were not evident prior to the strike," the coalition said in a statement.

Unintentional deaths also can occur when a civilian enters a bomb's zone of destruction after the 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 2,463 at a minimum.

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

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