More than 330,000 people have been killed in the Syrian conflict since it started six years ago, around a third of them civilians, a monitor said on Sunday.

The Britain-based Syrian Observatory for Human Rights said it has documented the deaths of 331,765 people across Syria since the conflict erupted in mid-March 2011 with anti-government protests.

Included in the overall death toll are 99,617 civilians, Observatory head Rami Abdel Rahman told AFP, adding that the figures were for the period between March 15, 2011 and July 15, 2017.

A total of 18,243 children and 11,427 women were among the civilians killed, the Observatory said. The figures were the latest provided by the Observatory since March when it said 320,000 people, including 96,000 civilians, had been killed.

In its latest report, the Observatory said a total of 116,774 members of the regime forces or regime supporters have been killed in Syria since the conflict began.

Of those, it said 61,808 were soldiers and 1,408 were members of the Lebanese Shia movement Hezbollah, an Iran-backed regime ally.

Also killed in Syria since the conflict erupted were 57,000 rebels, including from the US-backed Syrian Democratic Forces, an Arab-Kurdish alliance.

In addition, more than 58,000 militants, mainly from the militant Islamic State (IS) group and Al Qaeda's former Syria affiliate, were killed, said the Observatory, noting that the figure also includes foreign extremists.

The conflict broke out with peaceful anti-government protests but quickly turned into a fully fledged war involving a multitude of local, regional and foreign powers. The brutal conflict has destroyed much of the country's infrastructure and displaced millions of people, more than half of Syria's population.