JERUSALEM, Nov 24 (Reuters) - The release of hostages under a temporary truce between Israel and Hamas militants will not happen before Friday (Nov 24), Israel's national security adviser said on Wednesday night.

Israel and Hamas agreed early on Wednesday to a ceasefire in Gaza for at least four days, to let in aid and free at least 50 hostages held by militants in the Palestinian 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 had yet to be officially announced. An Egyptian security source said mediators sought a start time of 10am on Thursday.

Israel's public broadcaster Kan, citing an Israeli official, reported there was a 24-hour delay in the agreement because the deal was not signed by Hamas and mediator Qatar. The official said they were optimistic the agreement will be implemented when it is signed.

"Negotiations for the release of our captives are progressing and continue all the time," Israeli National Security Adviser Tzachi Hanegbi said in a statement released by the prime minister's office.

"The start of the release will proceed according to the original agreement between the parties, and not before Friday," Hanegbi said.

Prime Minister Benjamin Netanyahu made no mention of a potential delay in implementation of the agreement during a press conference late on Wednesday. Hanegbi's statement was released about an hour after the press conference.

The first truce in a near seven-week-old war, reached after mediation by Qatar, was hailed around the world as a sign of progress that could ease the suffering of civilians in Israel-besieged Gaza and bring more Israeli captives home. Arab ministers praised the agreement but said it should become a first step toward a full ceasefire.

Israel said the ceasefire could be extended further if more hostages were freed, and a Palestinian source said as many as 100 hostages in total could be released by the end of the month.

Hamas and allied groups captured around 240 hostages when gunmen rampaged through southern Israeli towns on Oct 7. Previously, Hamas had released just four.

Israel has subjected Hamas-ruled Gaza to a siege and relentless bombardment since the Oct 7 attack, which killed 1,200 people, mostly civilians, according to Israeli tallies. Since then, over 14,000 Gazans have been killed, around 40 per cent of them children, according to medical officials in the territory.

"It's not going to get all the hostages out, but it does get these first 50 or so, all women and children... We'll start to see them come start to get released over the next 24 hours or so," White House national security spokesperson John Kirby said.

The United States also hoped that hundreds of humanitarian aid trucks would reach Gaza in the next few days, Kirby said.

"Now, it's important that all aspects are fully implemented," U.S. President Joe Biden said in a comment on the deal on X.

The Red Cross will be able to visit any remaining hostages in Gaza, Israeli Prime Minister Benjamin Netanyahu said at a press conference on Wednesday night.

Photo from Reuters