At least 60,000 people have died in Syrian government jails during the past five years of war, a new report has found.
The Syrian Observatory for Human Rights report said tens of thousands of people had been killed by the regime of Bashar al Assad, either as a direct result of torture or denial of food and medicine.
The monitoring group said it had arrived at the number by adding up death tolls provided by sources in several Syrian jails and security agencies.
UN investigators said in February detainees held by the Syrian Government were being killed on a massive scale.
The report came as rebel groups vowed to no longer abide by a "cessation of hostilities" and set a 48-hour deadline for the US and Russian sponsors in Syria's conflict to halt a regime offensive in the Damascus region.
"We are giving the sponsors of the ceasefire 48 hours to rescue what remains of the accord and to force the criminal regime of [President] Assad and his allies to completely and immediately halt their brutal offensive against Daraya and Eastern Ghouta," 29 rebel groups said in a joint statement.
"In view of the regime's offensive against all the liberated regions, in particular Daraya … we consider the ceasefire accord to have totally collapsed.
"Rebel groups will take all possible measures and respond with all means to defend our people and on all fronts until the regime totally halts its offensives against all liberated regions, especially Daraya, and pulls back to its pre-May 14 positions."