BEIRUT (Reuters) - Over 146,000 people, more than a third of them civilians, have been killed in Syria's uprising-turned-civil war which enters its fourth year this month, the Syrian Observatory for Human Rights said on Thursday.

The last U.N. 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.

Reuters could not verify the death toll of 146,065 published by the UK-based Observatory, an anti-government group which uses a network of sources across Syria to document the violence there.

The revolt against President Bashar al-Assad began in March 2011 as peaceful street protests but transformed into an armed insurgency after a fierce security crackdown. It has descended into a civil war with sectarian dimensions.

The Observatory said it counted nearly 36,000 rebel fatalities, 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.

It said more than 56,000 of those killed were from pro-Assad forces, including 332 from the Lebanese Shi'ite Muslim group Hezbollah and 459 Shi'ite fighters from other countries.

But the group said the true toll on both sides was probably much higher - by perhaps more than 60,000.

Both sides try to hide their casualties, it said, making fighter death tolls very difficult to gauge.

(Reporting by Stephen Kalin; Editing by Robin Pomeroy)