A man carries the covered body of a boy following a reported barrel bomb attack on the Bab al-Nairab neighbourhood of the northern Syrian city of Aleppo on November 24, 2016. (AFP/ AMEER ALHALBI )

WASHINGTON, Dec. 2 (Xinhua) -- The U.S.-led coalition killed 54 civilians between March 31 and Oct. 22 in airstrikes against Islamic State (IS) targets in Syria and Iraq, raising the death toll of civilians to 173 since the start of the campaign in 2014, said the Pentagon on Thursday.

The civilian deaths occurred in six airstrikes in Syria and one airstrike in Iraq during the period, according to a Pentagon statement, which said some casualties were "unavoidable."

Three more reports received in October were still under assessment, said the Pentagon.

The most deadly airstrike occurred on July 18, 2016 near Manbij, Syria, which claimed 24 civilian lives while killing about 100 IS militants, it said.

The Pentagon also said the civilians killed were people "who had been interspersed with combatants" in a known IS staging area where no civilian was seen in the 24 hours before the strike.

The Pentagon's figures contradict the assessment by London-based Amnesty International, which estimated that about 300 civilians have been killed in 11 coalition airstrikes in Syria alone.