More than 65,000 have vanished in Syria since 2011, according to The Syrian Network for Human Rights.
Human rights groups have urged the international community to hold the Syrian government and opposition armed groups accountable for the tens of thousands of people who have disappeared during the country’s long-running civil war.
The call was made in a statement marking the International Day of the Disappeared on Wednesday.
“Amid the brutality and bloodshed of the Syrian conflict, the plight of those who have vanished after being arrested by the authorities or detained by armed groups is a tragedy that has been largely ignored internationally,” said Philip Luther, Amnesty International’s research and advocacy director for the Middle East and North Africa.
“Tens of thousands of families have been desperately trying to uncover the fate of their missing relatives,” he added.
According to the Syrian Network for Human Rights (SNHR), 75,000 people have been subjected to enforced disappearance by the Syrian government since the conflict began in 2011.
Human Rights Watch (HRW) said that a failure to address the issue means that more of the disappeared would likely be tortured or executed with each passing day.
“Syria will not be able to move forward if negotiations fail to adequately address the horrors of detention and disappearance,” said Sarah Leah Whitson, HRW’s Middle East director.
The group said that as part of any transitional process, international backers of negotiations to end Syria’s civil war should include the formation of an independent body to investigate the cases of thousands of disappeared Syrians.
The rights organisations also called on Russia and Iran, the most prominent backers of Syrian President Bashar al-Assad, to pressure his government to immediately publish the names of all individuals who died in Syrian detention facilities.
Furthermore, the government must be pressed to inform families of the deceased and to return the bodies to their relatives, the groups said.
The organisations also called on backers of non-state armed groups, such as Turkey and the United States, to make them reveal the fate of detainees in their custody and allow humanitarian agencies access to their detention facilities.
It is estimated that more than 2,000 Syrians are held by opposition groups and the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.
Many families who have lost loved ones due to forced disappearances suffer psychologically and may face months or years without learning of their relatives’ fate.