Syria's civil war has killed more than 120,000 people in 31 months, the Syrian Observatory for Human Rights said Thursday.

"We have documented the killing of 120,296 people since the start of the revolution in March 2011," said the Britain-based monitoring group.

Of the dead, 42,495 were civilians, including 6,365 children and 4,269 women, it said.

Another 25,699 rebel fighters were killed, as were 48,880 loyalists.

Among the opposition fighters, 18,122 were civilians who had taken up arms against President Bashar al-Assad's regime following a crackdown on Arab Spring-inspired protests in the spring of 2011.

Another 2,022 were soldiers who had defected from the army to join the rebellion, and 5,375 were foreign jihadists who had travelled to Syria to fight Assad's troops.

Among the loyalists, 29,954 were army soldiers, while 18,678 were members either of the pro-regime popular committees or the paramilitary National Defence Force.

Another 187 were members of Lebanon's powerful Shiite Hezbollah, who had joined the war in support of Assad. An additional 61 non-Syrian, pro-regime fighters have been killed.

The Observatory also reported the killing if 2,772 unidentified people.

The group, which relies on a broad network of doctors, lawyers and activists across Syria for its reporting, said the actual toll of the conflict is likely to be higher, and that the fate of thousands remains unknown.

Among the disappeared are "more than 10,000 detainees and missing in the regime's jails, as well as more than 3,000 soldiers held prisoner by the rebel groups," the Observatory said.

Hundreds of civilians have also been kidnapped.

Syria's war began as a protest movement demanding the fall of Assad, but the conflict spiralled into a civil war after Damascus launched a brutal crackdown on dissent.