U.S. President Donald Trump said on Monday that if all the hostages held in Gaza are not returned by Saturday at noon he would propose canceling the Israel-Hamas ceasefire and letting "all hell break loose."

Speaking to reporters in the Oval Office, Trump also said he might withhold aid to Jordan and Egypt if they don't take Palestinian refugees being relocated from Gaza.

Earlier on Monday, Hamas announced it would stop releasing Israeli hostages until further notice over what the Palestinian militant group called Israeli violations of a ceasefire agreement in Gaza, raising the risk of reigniting the conflict.

Hamas was to release more Israeli hostages on Saturday in exchange for Palestinian prisoners and other Palestinians held in Israeli detention as had happened over the past three weeks.

After Hamas' unexpected announcement, hostage families and their supporters packed the area of Tel Aviv now known as Hostages Square on Monday night to press the government not to abandon the deal.

"Every single person that doesn't belong there needs to come home now," said Shoshana Brickman, a protester who joined a crowd of around 2,000 in the unscheduled demonstration. "Every single person, all the hostages, all of them."

Hamas said it made its announcement five days before Saturday's scheduled hostage release so mediators could pressure Israel to uphold its ceasefire obligations and "keep the door open for the exchange to take place on time."

Israeli Defence Minister Israel Katz said Hamas' move violated the ceasefire and he instructed the military to be at the highest level of readiness in Gaza and for domestic defence.

An Israeli official said Prime Minister Benjamin Netanyahu would meet with the security cabinet which includes defence, national security and foreign affairs ministers on Tuesday morning.

Two Egyptian security sources told Reuters on Monday mediators fear a breakdown of the ceasefire agreement. Qatar and Egypt brokered the deal alongside the United States.

So far, 16 of the 33 hostages to be freed in the first 42-day phase of the deal have come home, as well as five Thai hostages who were returned in an unscheduled release.

In exchange, Israel has released hundreds of prisoners and detainees, including prisoners serving life sentences for deadly attacks and Palestinians detained during the war and held without charge.

HOUSING AN ISSUE

Hamas military wing spokesperson Abu Ubaida said Israeli ceasefire violations included delaying Palestinians from returning to northern Gaza, shelling and firing upon Palestinians and stopping humanitarian aid from entering the strip as stipulated under the ceasefire.

In turn, Israel has accused Hamas of not respecting the order in which the hostages were to be released and of orchestrating abusive public displays before large crowds when hostages have been handed over to the Red Cross.

Aid organizations have said that the flow of humanitarian assistance to Gaza has risen since the ceasefire took effect, and Israel denies Hamas' claim that it impedes the flow of aid.

An official with knowledge of the ceasefire negotiations told Reuters that Israel had rejected requests by the United Nations, Qatar and others to allow temporary housing units to be brought into Gaza to shelter displaced people as required under the ceasefire agreement.

Hamas officials said Israel had blocked entry of 60,000 mobile houses and 200,000 tents as well as heavy machinery to remove rubble and fuel.

U.S. President Donald Trump's statements last week that Palestinians should be moved out of Gaza, leaving the coastal enclave to be developed as a waterfront real estate project under U.S. control, have upended expectations for the postwar future.

Trump said Palestinians would not have the right of return to Gaza, according to an excerpt of a Fox News interview released on Monday.

"I'm talking about building a permanent place for them" because "it’ll be years before (Gaza is) habitable," he said. Trump said he thought he could make a deal with Egypt and Jordan to take Palestinians from Gaza.

On his return to Israel over the weekend from Washington, Netanyahu praised Trump's ideas.

Netanyahu's office said on Monday that an Israeli delegation returned from ceasefire talks in Qatar, amid growing doubts over the Egypt- and Qatar-brokered process to end the war.

There was no immediate explanation for the Israelis' return. The talks are intended to agree the basis for a second stage of the multi-phase ceasefire and hostage-for-prisoner exchange accord reached last month.

A Palestinian official close to the discussions said progress was being held up by mistrust between the two sides.

Israelis were shocked by the emaciated appearance of Ohad Ben Ami, Eli Sharabi and Or Levy, three hostages who were released on Saturday, which has complicated the picture.

An Israeli Channel 13 poll showed on Monday that 67% of Israelis wanted to move to the next phase of the deal while 19% did not. The poll was taken before Hamas announced it was postponing the process.