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.

The six Israelis were the last living captives set to be released in the first phase of the cease-fire. The agreement, strained for weeks, has an uncertain future.

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.