Over 350,000 of the casualties have been confirmed while the remaining victims haven't yet been identified according to a UK-based war monitor, the Syrian Observatory for Human Rights, Reuters reported on Thursday.
According to the Observatory, at least 85 percent of the casualties were civilians killed by the Syrian regime and its allies, which has been accused of using chemical weapons.
The conflict in Syria began after mass protests in March 2011 erupted into a civil war between pro-Syrian regime forces and rebel factions.
The rise of ISIS across Iraq and Syria in 2014 further complicated the situation and regional and global powers were drawn into the conflict.
At least half of the pre-war population has been displaced at one time or another, with many fleeing the country altogether.
Turkey has been a main backer of the opposition groups and rebels trying to remove Syrian President Bashar Assad from power while Russia and Iran have been among his main allies.
As Syria descended into civil war, Kurds carved out the self-autonomous region of Rojava. They have generally maintained an uneasy truce with Assad.
The US-led anti-ISIS coalition has been working with the mainly Kurdish Syrian Democratic Forces (SDF) to oust the terror group.
Their forces have defeated ISIS in Manbij, Tabqa, and Raqqa and are currently battling militants in eastern Deir ez-Zor province where they have occasionally clashed with forces loyal to the Syrian regime.
The Kurdish YPG forces announced on Tuesday that they were withdrawing the last of their advisors from Manbij.
That announcement came a day after Washington and Ankara said they had reached an agreement on a road map for the future of Manbij that aims to prevent a Turkish offensive on Kurdish forces in the city.
Turkey considers the YPG a branch of the PKK, a named terror organization and has long railed against American backing for the Kurds in the war against ISIS. The YPG denies any organizational links to the PKK.
Three guarantor countries in the peace process, Turkey, Iran and Russia, teamed up to help mediate a peace settlement for Syria's conflict that has caused the worst refugee crisis since World War II.
Multiple rounds of UN-led talks have made little progress.
