Human Rights Watch called Wednesday for an independent enquiry to probe the fate of thousands of people who have disappeared in Syria's war and to identify mass graves.
The Syrian war began in March 2011 when waves of peaceful demonstrations were repressed by the security forces, eventually leading to an all-out conflict involving many sides.
More than 330,000 people have been killed and millions displaced, but HRW says the exact number of those missing could not be determined as most detention facilities were off-limits to outsiders.
"An independent institution in charge of investigating the fate and whereabouts of the disappeared, as well as unidentified human remains and mass graves in Syria, should be created immediately," said HRW.
The New York-based rights watchdog issued the appeal in a statement coinciding with the International Day of the Victims of Enforced Disappearances.
Last year, a team of UN experts raised the alarm over enforced disappearances and called for the situation to be referred to the International Criminal Court.
"Syria will not be able to move forward if negotiations fail to adequately address the horrors of detention and disappearance," said Sarah Leah Whitson, Middle East director at HRW.
"This should not be ignored. Without progress, each day that passes will likely see more of the disappeared tortured or executed," she said.
The watchdog said the enquiry "should have a broad mandate to investigate, including by reviewing all official records and interviewing any official".
Thousands of peace activists were arrested during the first years of the Syrian conflict, and some are still languishing in prisons, according to non-governmental organizations.
Earlier this month, the death of prominent computer scientist Bassel Khartabil Safadi was confirmed two years after his execution by the regime.
Safadi had been arrested in March 2012 in the wake of the crackdown.