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.

Israel said on Sunday it was delaying the release of hundreds of Palestinian prisoners it had planned to free the day before until militant group Hamas met its conditions, underscoring the fragility of the Gaza ceasefire accord.

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.