GENEVA - Detainees held by the Syrian government are dying on a massive scale amounting to a state policy of "extermination" of the civilian population, a crime against humanity, United Nations investigators said on Monday.The UN commission of inquiry called on the Security Council to impose "targeted sanctions" on Syrian officials in the civilian and military hierarchy responsible for or complicit in deaths, torture and disappearances in custody, but stopped short of naming them.In their report, the independent experts said they had also documented mass executions and torture of prisoners by two jihadi groups, Jabhat al-Nusra and Islamic State (ISIS), constituting war crimes.The report, "Out of Sight, Out of Mind: Deaths in Detention", covers March 10, 2011 to November 30, 2015. It is based on interviews with 621 survivors and witnesses and evidence gathered by the team led by chairman Paulo Pinheiro.