More than 14,000 people have been killed in Israeli strikes in Gaza since war began.
Israeli officials said there would be no pause in the Gaza fighting or any release of hostages held by Hamas before Friday, despite the two sides agreeing a truce.
The agreement was approved by the Israeli government early Wednesday and had been widely expected to take effect on Thursday.
But an Israeli official told AFP early Thursday there would be no halt in the fighting between Israel and Gaza's Hamas rulers "before Friday".
The official's comments came shortly after national security adviser Tzachi Hanegbi said late Wednesday that none of the hostages seized in the Hamas attacks on southern Israel on October 7 would be freed before Friday.
"The contacts on the release of our hostages are advancing and continuing constantly," he said in a statement.
"The start of the release will take place according to the original agreement between the sides, and not before Friday."
Minutes earlier, Israeli officials notified journalists that a media centre would open in Tel Aviv at noon on Thursday "to cover the return of the hostages".
Hanegbi gave no further details, and Israeli officials did not immediately offer any explanation for the developments.
Under the deal, Israel and Hamas agreed on a four-day truce in the Gaza war during which the Palestinian militants would free at least 50 of the hostages taken in their deadly attacks in a series of daily batches.
In turn, Israel would release at least 150 Palestinian prisoners and allow more humanitarian aid into the coastal territory after more than six weeks of bombardment, heavy fighting and a crippling siege.
The hostages to be freed in the process are women and children, and the Palestinian prisoners are women and teenage boys aged 18 and under.
The war started after Hamas militants on October 7 launched the worst attack in Israel's history, killing about 1,200 people, mostly civilians, and taking around 240 hostages, Israel says.
Israel launched a major bombing campaign and then a ground offensive in Gaza which Hamas says has killed 14,100 people, thousands of them children.
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