The Syrian state and allied militia have detained and abducted tens of thousands of people since 2011 in a campaign that is a crime against humanity, according to Amnesty.
The rights organisation has interviewed relatives of the disappeared, who said they had been forced to pay bribes to gain information on the fate of their family members.
Amnesty said it had attempted to discuss the issue with the Syrian authorities and was awaiting a response. The Syrian government has regularly dismissed reports accusing the state of human rights abuses, writes The Guardian.
â€œThe enforced disappearances carried out since 2011 by the Syrian government were perpetrated as part of an organised attack against the civilian population that has been widespread, as well as systematic,â€ Amnesty said.
It described such acts as crimes against humanity and called on Damascus to allow access to international monitors from the United Nations-mandated Independent International Commission of Inquiry for Syria to seek information on detainees.
More than 65,000 people, most of them civilians, were forcibly disappeared between March 2011 and August 2015 and remained missing, Amnesty said, citing figures from the Syrian Network for Human Rights, a Syria-based monitoring group.
Over 220,000 people have died in the conflict.