During their deadly 2023 attack on Israel, Hamas-led militants captured more than 250 people, according to Israeli tallies, including dual Israeli-American nationals.
Around half of the 101 foreign and Israeli hostages still held incommunicado in Gaza are believed to be alive.
Making his most explicit comments on the fate of the hostages since his election in November, Trump said on social media:
"(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 added: "Those responsible will be hit harder than anybody has been hit in the long and storied History of the United States of America."
Hamas has called for an end to the war and total Israeli withdrawal from Gaza as part of any deal to release remaining hostages.
Israeli Prime Minister Benjamin Netanyahu has said the war will go on until Hamas is eradicated and poses no more threat to Israel.
On Monday, Hamas said that 33 hostages in Gaza had been killed during the almost 14-month-old war between the Palestinian militant group and Israel in the enclave, without giving their nationalities.
Israel launched its war after Hamas-led fighters attacked Israeli communities on October 7, 2023, killing 1200 people, according to Israeli tallies.
Israel's military offensive has killed more than 44,400 Palestinians and displaced most of Gaza's population, Gaza officials say. Vast swathes of the enclave lie in ruins.
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