Israel has said one of four bodies it received from Hamas on Tuesday evening does not match any of the hostages that were in the group's custody.
The country’s military said the discovery was made following examinations at the National Institute of Forensic Medicine.
"Hamas is required to make all necessary efforts to return the deceased hostages," the military said on Wednesday.
The four bodies were handed over after an Israeli military agency warned it would slash aid deliveries to Gaza as the militant group was not returning remains as agreed under the ceasefire deal.
By Monday night, Hamas had released four bodies, and four more followed late on Tuesday.
Of that second group of four bodies, three were identified, as Uriel Baruch, Tamir Nimrodi and Eitan Levi.
The other is not a hostage, Israel has said.
Israeli Prime Minister Benjamin Netanyahu demanded that Hamas fulfills the requirements laid out in the ceasefire deal about the return of the hostages' bodies.
"We will not compromise on this and will not stop our efforts until we return the last deceased hostage, until the last one," he said.
Israel reportedly U-turned on its threat to limit humanitarian aid into Gaza after the four bodies were returned.
Planned measures against Hamas that included halving the number of aid lorries entering the enclave by keeping the Rafah crossing between Gaza and Egypt shut have been cancelled, Israeli public broadcaster Kan reported on Wednesday.
Israel has reportedly decided to proceed with opening the Rafah crossing after all.
Israel announced it would cut in half the number of humanitarian aid lorries allowed into Gaza in a move to punish Hamas for what it called the militant group's violation of its agreement to transfer remains under the ceasefire deal reached last week.
Meanwhile, re-emergent Hamas fighters demonstrated they were reasserting control in Gaza by deploying hundreds of security forces in the streets and executing several people they accused of collaborating with Israel.
The outlook for Trump's peace plan darkened further when the president threatened Hamas with military strikes unless the militant group disarmed.
"If they don't disarm, we will disarm them. And it will happen quickly and perhaps violently," Trump said at the White House, one day after he spoke before the Knesset in Jerusalem.
Israeli Prime Minister Benjamin Netanyahu has maintained the war cannot end until Hamas surrenders its weapons and cedes control of Gaza, a demand the fighters have so far rejected.
On Monday, Trump proclaimed the "historic dawn of a new Middle East" to Israel's parliament, as Israel and Hamas were exchanging the last 20 living Israeli hostages in Gaza for nearly 2,000 Palestinian detainees and prisoners.
But the return of 28 dead hostages has remained one of the final details to be worked out in the deal to end more than two years of conflict that began with Hamas attacks on Israel on October 7, 2023.
Hamas has now handed over eight coffins of dead hostages, leaving at least 19 presumed dead and one unaccounted for still in the Gaza Strip.
On Tuesday, the Israeli military said it received four coffins from the Red Cross at a meeting point in northern Gaza Strip. Those coffins, escorted by Israeli forces, crossed the border into Israel shortly before midnight (10pm BST) and were being taken for forensic identification, the Israeli military said.
“At this moment, the men are continuing to oversee the implementation of what was agreed upon regarding the handover of bodies as part of the agreement to end the war in Gaza,” Hamas spokesperson in Gaza Hazem Qassem on Facebook.
Hamas, which seized Gaza in a brief 2007 civil war, has swiftly reclaimed the streets of Gaza's urban areas following the partial withdrawal of Israeli troops last week. Gaza residents said Hamas fighters were increasingly visible on Tuesday, deploying along routes needed for aid deliveries.
In a video circulated late on Monday, Hamas fighters dragged seven men with hands tied behind their backs into a Gaza City square, forced them to their knees and shot them from behind, as dozens of onlookers watched from nearby storefronts.
A Hamas source confirmed that the video was taken on Monday and that Hamas fighters participated in the executions.
Trump has previously given his blessing to Hamas to reassert some control of Gaza, at least temporarily. Israeli officials have so far refrained from commenting publicly on the re-emergence of the group's fighters.
Palestinian security officials said dozens of people had been killed in clashes between Hamas fighters and rivals in recent days.
In addition Israel, using aerial drones, killed five Palestinians as they went to check on houses in a suburb east of Gaza City, and an Israeli airstrike killed one person and injured another near Khan Younis, Gaza health authorities said.
Hamas accused Israel of violating the ceasefire. The Israeli military said it had fired on people who crossed truce lines and approached its forces after ignoring calls to turn back.
he ceasefire has stopped two years of devastating warfare in Gaza triggered by the October 7, 2023 attack in which Hamas-led gunmen killed around 1,200 people and took 251 hostages back to Gaza.
Israel's military has killed at least 67,000 people in Gaza according to local health authorities, with thousands more feared dead under the rubble.
