The conflict in Syria has left more than 220,000 dead since it began four years ago with an uprising against President Bashar al-Assad, a key monitoring group said Thursday.

"We have counted 222,271 deaths since the start of the revolt in March 2011," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.

Based in Britain, the Observatory uses a broad network of sources on the ground in Syria to gather information about the conflict.

Abdel Rahman said more than 67,000 of the dead were civilians, including more than 11,000 children.

Among dead combatants, nearly 47,000 were from pro-regime forces, including more than 3,000 foreign fighters. Nearly 700 were fighters with Lebanon's Shiite Hezbollah movement, a key backer of Assad.

Nearly 40,000 anti-regime fighters and about 28,000 foreign jihadists have been killed in the conflict, the Observatory said.

Foreign jihadists have flocked to join the conflict in Syria, fighting for the Islamic State group or other groups such as the Al-Qaeda affiliated Al-Nusra Front.

The toll does not include some 20,000 people listed as missing.

The Observatory says the full death toll is likely to be much higher than the deaths it has been able to count.