A planned release of 50 hostages held in Gaza along with 150 Palestinian prisoners and a pause in fighting has been delayed hours before it was set to begin on Thursday, Israeli, U.S. and regional officials said.

“The start of the release will take place according to the original agreement between the sides, and not before Friday,” Israel’s national security adviser Tzachi Hanegbi said Wednesday. Hanegbi said contacts toward the release of the hostages continued.