Syria's civil war has killed more than 200,000 people in less than four years, a monitoring group says, adding that most were fighters from the two sides.

"We have documented the killing of 202,354 people since March 2011," Syrian Observatory for Human Rights director Rami Abdel Rahman said.

He said more than 130,000 of them were combatants.

"Of the total, 63,074 of the killed were civilians, including 10,377 children," Mr Abdel Rahman said.

"Among the anti-regime fighters, 37,324 were Syrian rebels, while 22,624 were non-Syrian jihadists.

"On the regime side, there were 44,237 soldiers, 28,974 members of the (paramilitary) National Defence Force, 624 members of (Lebanon's Shiite) Hezbollah, and 2,388 pro-regime Shiite fighters from beyond Syria and Lebanon."

Another 3,011 were unidentified, he said.

Mr Abdel Rahman said the toll "is probably much higher than 200,000, but certain areas under regime and Islamic State group control are impossible to work in freely".

He also said some 300,000 people are being held in Syria's infamous jails, including 20,000 who have gone missing entirely.

Thousands of other people - combatants and civilians - have also been taken hostage by IS militants and other factions active in Syria.

Mr Abdel Rahman blamed the international failure to refer Syria to the International Criminal Court for the constant bloodletting.

"By failing to take the killers to court, the international community gives them the implicit go-ahead to keep on killing," he said.

There have been several attempts to refer Syria to the ICC, but Russian and Chinese vetoes at the UN Security Council have blocked them.

AFP