A Belgian F-16 fighter jet pilot gestures while taking off to join an international coalition fighting against ISIS.

Air strikes by U.S.-led forces in Syria have killed 865 people, including 50 civilians, since the start of the campaign in late September against Islamic State of Iraq and Syria (ISIS) militants, a group monitoring the war said on Wednesday.

The Britain-based Syrian Observatory for Human Rights said the majority of the deaths, 746, were ISIS fighters and that the actual figure could be much higher. ISIS has seized tracts of territory in Syria and neighboring Iraq, where it has also been targeted by U.S.-led forces since July.

Eight of the civilians killed were children, the Observatory said. It said 68 members of al-Qaeda-affiliated Nusra Front were also killed in the air strikes, which started early on Sept. 23.

Coalition strikes have hit the Syrian provinces of Aleppo, Deir al-Zor, Hasaka, Raqqa and Idlib, the Observatory said.

The United States has said it takes reports of civilian casualties seriously and says it has a process to investigate each allegation.

Washington justified its action in Syria under Article 51 of the U.N. Charter, which covers an individual or collective right to self-defense against armed attack.

Around 200,000 people have been killed in Syria’s conflict, which is now in its fourth year, according to the United Nations.

 
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