Israel has delayed an expected release of hundreds of Palestinian prisoners and detainees in exchange for returned Israeli hostages “until further notice,” the Palestinian Prisoners Society said early Sunday.

Hamas freed six hostages from Gaza on Saturday, the last living Israeli captives slated for release under the first phase of a fragile ceasefire accord, in exchange for hundreds of Palestinian prisoners and detainees.

In what was set to be the largest swap under January’s ceasefire agreement, six hostages were freed, but the release of some 600 Palestinian prisoners was delayed. It comes after Shiri Bibas’s body was returned by Hamas.