Washington DC - Donald Trump on Monday warned of massive repercussions if hostages in Gaza are not released by the time he takes office.

The threat from the US President-elect comes after exhaustive diplomacy by outgoing President Joe Biden's administration that has so far failed to secure a deal that would both end Israel's war in Gaza and free hostages seized 14 months ago.

"If the hostages are not released prior to January 20, 2025, the date that I proudly assume Office as President of the United States, there will be ALL HELL TO PAY in the Middle East, and for those in charge who perpetrated these atrocities against Humanity," Trump wrote on his Truth Social platform.

"Those responsible will be hit harder than anybody has been hit in the long and storied History of the United States of America. RELEASE THE HOSTAGES NOW!"

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Trump has vowed staunch support for Israel and to dispense with Biden's occasional criticism, but has also spoken of his desire to secure deals on the world stage.

Hamas, the militant group which controls Gaza, staged the deadliest-ever attack on Israel on October 7, 2023. The assault resulted in 1,208 deaths, mostly civilians, according to an AFP tally of Israeli official figures.

Hamas militants seized 251 hostages during the attack, some of whom were already dead. Of those, 97 are still believed to be held in Gaza, including 35 the army says are dead.

Israel's retaliatory campaign has killed at least 44,429 people in Gaza, according to figures from the territory's health ministry that the United Nations considers reliable, with the total number believed to be orders of magnitude higher.

The UN chief said on Monday the situation in Gaza is "appalling and apocalyptic," warning that the horrors waged by Israel on Palestinians and civilians in the territory are "reminiscent of the gravest international crimes."