A damaged classroom is pictured in the rebel held besieged city of Douma, in the eastern Damascus suburb of Ghouta, Syria March 2, 2017. REUTERS/Bassam Khabieh

WASHINGTON (Reuters) - Twenty-one civilians were killed in nine separate strikes by the U.S.-led coalition fighting Islamic State in Iraq and Syria between November and January, the U.S. military said on Saturday.

That brings the total number of civilians unintentionally killed by the coalition since the start of operations against the militant group in 2014 to 220, the military said in a statement.

“Although the Coalition takes extraordinary efforts to strike military targets in a manner that minimizes the risk of civilian casualties, in some incidents casualties are unavoidable,” the statement said.

The military estimate is far lower than those provided by monitoring groups.

At least 2,463 civilians have been killed by coalition air strikes, according to monitoring group Airwars.

In an incident on Jan. 13, near the Iraqi city of Mosul, the military said eight civilians were killed during a strike on Islamic State fighters in a house.

“During post-strike video analysis civilians were identified near the house who were not evident prior to the strike,” the statement said

As of Feb. 28, the coalition has carried out 18,666 strikes in Iraq and Syria since the start of the operation, according to U.S. military data. The average daily cost of operations is $12.7 million, according to the data.