The Syrian Observatory for Human Rights, a British-based war monitor, said on Monday about 511,000 people had been killed in the Syrian war since it began seven years ago.
The Observatory, which tracks death tolls using a network of contacts inside Syria, said it had identified more than 350,000 of those killed, and the remainder were cases where it knew deaths had occurred but did not know the victims' names.
The conflict began after mass protests on March 15 2011, dragging in regional and global powers and forcing millions of people - more than half the pre-war population - to flee their homes.
>> Israel and Iran back on collision course in the north | Analysis
About 85 percent of the dead were civilians killed by the forces of the Syrian government and its allies, the Observatory said. The Syrian military, joined by its ally Russia since 2015, has used air power extensively.
The Russian military said late on Sunday that it had managed to evacuate 52 civilians, including 26 children, from Syria’s rebel-held eastern Ghouta after talks with local authorities.
- Sex for Food: With Nowhere Else to Turn, Women of Syria's Aleppo Face Exploitation by Aid Workers
- U.S. Warns Syria: 'Very Unwise' to Use Chemical Weapons
- Can Syrian Carnage Be Compared With the Shoah? U.S. Holocaust Museum Insists It Must
The civilians, inhabitants of the town of Misraba, were taken to a temporary refugee camp where they were receiving medical aid, the Russian military said in a statement.
Jaish al-Islam, one of the main rebel groups in Syria's eastern Ghouta, said on Monday it had reached an agreement with the government's ally Russia to evacuate wounded people from the besieged enclave near Damascus.
The rebel group said it communicated with Russia through the United Nations to reach the agreement, as the government presses a major offensive against the enclave with Russian military help.
On February 24, The United Nations Security Council adopted a resolution demanding a 30-day truce in Syria to allow aid deliveries and medical evacuations with the support of Syrian ally Russia.
The vote at the United Nations came as warplanes pounded eastern Ghouta, the last rebel enclave near Syria's capital, for a seventh straight day while residents holed up in basements.
On February 25, Syrian rebels said they clashed with pro-government forces in the early morning, despite the UN resolution. Rescuers and residents said warplanes struck some towns in the eastern Ghouta enclave.
On February 26, Russian President Vladimir Putin ordered the implementation of a daily cease-fire in Syria's eastern Ghouta and the creation of a "humanitarian corridor" via which civilians can leave, according to Russian Defense Minister Sergey Shoygu.