Human rights groups called all parties in Syria to take action on people who went missing over the eight years of civil war and urged the international community to pay more attention to the issue.
"Tens of thousands have been forcibly disappeared by the Syrian government; and many have gone missing after being abducted by armed anti-government groups or the Islamic State [Daesh]," said eight Syrian and international rights groups in a statement.
"Hundreds have died in detention of torture or ill-treatment," said the statement, whose signatories included Amnesty International and Human Rights Watch.
The Britain-based Syrian Observatory for Human Rights estimates that around 200,000 people have gone missing since the start of the conflict in 2011, with nearly half believed to be held in government prisons. Many have been subjected to "staggering levels of serious violations" committed by the regime and anti-regime forces, including extrajudicial executions, torture and arbitrary detention, the report said.
They include activists, lawyers, journalists and humanitarian aid workers, some of whom were perceived to oppose the Bashar Assad regime, it added.
The report called on member states of the U.N. Security Council to "end the suffering of the families of the disappeared and of the arbitrarily detained" by putting "pressure" on the culprits and their international backers to reveal the names, fate and whereabouts of the thousands concerned.
It also called on the international community to "support the creation of a unified system for logging all cases of missing persons in Syria." Rarely told where their loved ones are held or whether they are still alive, the relatives of the missing often have to shuffle between different security agencies in a desperate bid to collect information.
As of May 2018, the Syrian regime started updating its civil records with the names of those who have died in its custody and in some cases provided families with death certificates that date as far back as 2013, the report said.
"However, the government has not responded to requests by families of detainees to obtain information on the circumstances of the enforced disappearances or the causes of death, or to take possession of the remains of those who died," said the report.
"As things stand, there is no way to verify the deaths without the government returning the remains to the families," it added.
The civil war in Syria has killed more than 370,000 people and displaced millions since it started with the brutal repression of anti-government protests in 2011.