At least 528,500 people were killed in the civil war in Syria, according to the Syrian Observatory for Human Rights (SOHR).
The figure includes thousands who were only recently confirmed dead.
Access to prisons and mass graves has become easier since rebel groups toppled the regime of President Bashar al-Assad in December.
According to the British-based SOHR, 6,777 people, more than half of them civilians, were killed in fighting in Syria last year.
The civil war in the country broke out after government forces in March 2011.