Rights groups Monday (May 13th) urged the international community to pressure all sides in Syria's war to reveal the fate of "tens of thousands" of people gone missing over years of conflict, AFP reported.

"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 of Iraq and Syria' (ISIS)," eight Syrian and international rights groups said 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 Syrian Observatory for Human Rights estimates 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 subject to "staggering levels of serious violations" committed by regime and opposition forces, including extrajudicial executions, torture and arbitrary detention, the report said.

They include activists, lawyers, journalists and humanitarian aid workers, it said.

The report called on member states of the UN 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 those 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".

In May 2018, the Syrian government 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.