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.
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.
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