The release of hostages under a temporary truce between Israel and Palestinian Hamas militants will not happen before Friday, Israel's national security adviser and the U.S. said, dashing hopes of relatives that some would be freed on Thursday.

Israel and Hamas agreed early on Wednesday to a ceasefire in the Gaza Strip for at least four days, to let in humanitarian aid and free at least 50 hostages held by militants in the enclave in exchange for at least 150 Palestinians jailed in Israel.

The starting time of the truce and release of hostages captured by Hamas during its Oct. 7 attack on Israel is yet to be officially announced. An Egyptian security source said mediators had sought a start time of 10 a.m. on Thursday.