The Hamas announcement that it will delay the next planned release of Israeli hostages from Gaza has threatened a fragile truce that’s seen as having the potential to wind down 16 months of war.

Hamas said delayed the release of the next group of Israeli hostages, accusing Israel of preventing aid and Palestinians from returning home in Gaza. Israel said Hamas is violating the ceasefire.

Stalling the next release of hostages from the Gaza Strip, scheduled for the coming weekend, raises new challenges for the already tenuous six-week truce and chances for a lasting end to the war.