The release of hostages under a temporary truce between Israel and Palestinian Hamas fighters will not happen before Friday, Israel’s national security adviser said.

Israel and Hamas agreed early on Wednesday to a ceasefire in Gaza for at least four days, to let in humanitarian aid and free at least 50 hostages held by Hamas 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 has yet to be officially announced. An Egyptian security source said mediators had sought a start time of 10 a.m. (0800 GMT) on Thursday.

“The negotiations on the release of our hostages are advancing and continuing constantly,” Israeli National Security Adviser Tzachi Hanegbi said in a statement released by the prime minister’s office.

“The start of the release will take place according to the original agreement between the sides, and not before Friday,” it said.

Also, White House spokesperson Adrienne Watson said final logistical details for the release were being worked out. “That is on track and we are hopeful that implementation will begin on Friday morning,” Watson said.

But there was no let-up in the fighting early on Thursday, reports said.

According to Reuters, Palestinian media said Israeli aircraft and artillery struck Gaza’s southern city of Khan Younis in at least two waves and 15 people were killed. Attacks were also reported in several other parts of Gaza, including the Jabalia and Nuseirat camps.

Israeli media, citing anonymous officials, reported the pause in fighting with Hamas also would not start before Friday. Israel’s Ynet news website reported that Israel had not yet received the names of the hostages slated for release by Hamas.