The UK-based Observatory, which monitors violence in Syria through a network of activists and medical or security sources, said that real toll was likely to be significantly higher at around 220,000 deaths.
Efforts to end the conflict by bringing together representatives of President Bashar Assad's government and the opposition have so far failed. The United Nations peace mediator for Syria said last week that talks were unlikely to resume soon.
The last UN figures, released in July 2013, put the death toll at least 100,000 but the United Nations said in January it would stop updating the toll as conditions on the ground made it impossible to make accurate estimates.
The Observatory said it had registered the deaths of 150,344 people since March 18, 2011, when Assad's security forces first fired on protesters calling for reform.
The Observatory said nearly 38,000 rebels were killed including fighters from the Nusra Front, al Qaeda's affiliate in Syria, and the Islamic State of Iraq and the Levant, an al Qaeda splinter group which includes many foreign fighters.
More than 58,000 pro-Assad fighters were killed, including regular security forces and Syrian pro-government militia, as well as 364 fighters from the Lebanese Shi'ite militia Hezbollah and 605 other foreign Shi'ite Muslims.In addition to the fatalities, the Observatory said 18,000 people were missing after being detained by security forces, while another 8,000 people had been detained by rebels forces or kidnapped.