Airstrikes by the U.S.-led coalition in Syria have killed 2,079 people, including 66 civilians, since the start of the aerial campaign against Islamic State militants last September, a group monitoring the war said on Thursday.

The Britain-based Syrian Observatory for Human Rights said a majority of the deaths, some 1,922, were Islamic State (ISIS) fighters.

The hardline group has seized tracts of territory in Syria and neighboring Iraq, where it has also been targeted by U.S.-led forces since last July.

Ten of the civilians killed were children and six were women, the Observatory said. It said 90 members of the al-Qaida-affiliated Nusra Front were also killed in the airstrikes, which started on Sept. 23.

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 220,000 people have been killed in Syria's conflict, which is now in its fourth year, the United Nations has said.