At least 42,000 people have been killed in violence since an uprising broke out against the rule of Syrian President Bashar al-Assad in March last year, a monitoring group said on Thursday.

"At least 29,455 civilians have been killed, as have 1,426 troops who defected to the opposition and 10,551 soldiers," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.

"An additional 652 people whose identity we have been unable to identify have been killed in the conflict," Abdel Rahman said. "A total of 42,084 people have died in the past 21 months."

The conflict started as a peaceful protest movement but escalated into an armed rebellion when the authorities used deadly force against demonstrators.

The Observatory counts non-army combatants who have taken up arms against the regime as civilians.

"When the crisis comes to an end, it is likely that we will find many more have been killed, because many thousands are missing in Syria's jails," Abdel Rahman said.

In addition, neither the army nor the rebels are willing to reveal their full casualty lists, he said. "That is part of their propaganda war."