(Reuters) - Air strikes by U.S.-led forces have killed 553 Islamist fighters and 32 civilians during a month-long campaign in Syria, a monitoring group which tracks the violence said on Thursday.

The Britain-based Syrian Observatory for Human Rights said the vast majority of the deaths, 464, were militants from Islamic State, an al Qaeda offshoot which has grabbed large areas of Syriaand neighboring Iraq. 

The attacks also killed 57 members of the al Qaeda-affiliated Nusra Front, the Observatory said. Six of the civilians were children and five were women, it added.

The United States has been carrying out strikes in Iraq against Islamic State since July and in Syria since September with the help of Arab allies. Britain and France have also struck Islamic State targets in Iraq.

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.

U.S. Central Command spokesman Colonel Patrick Ryder said on Saturday that Washington took "reports of civilian casualties or damage to civilian facilities seriously and we have a process to investigate each allegation."

Close to 200,000 people have been killed in Syria's three-year civil war, according to the United Nations.

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

(Reporting by Oliver Holmes; Editing by Andrew Heavens)