More than 40,000 people have been killed in 20 months of conflict between Syrian President Bashar al Assad's forces and opposition fighters, a violence monitoring group said.

Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights, said about half of the fatalities were civilians and the other half split about evenly between rebels and government soldiers.

"The figure is likely much higher as the rebels and the government lie about how many of their forces have died to make it look like they are winning," he said.

The figures cannot be independently confirmed due to severe reporting restrictions imposed by Syria.