Israeli Prime Minister Benjamin Netanyahu vowed revenge on Hamas on Thursday after the group released what it said were the bodies of four Israeli hostages, including the infant Kfir Bibas and his four-year-old brother Ariel, the youngest of those abducted during the October 7, 2023 attack.

Palestinian militants handed over four black coffins in a carefully orchestrated public display, with dozens of armed Hamas militants and crowds of Palestinians gathering to watch.

United Nations human rights chief Volker Turk criticized the handoff, saying that the parading of bodies was abhorrent and cruel and that it flew in the face of international law.

"Under international law, any handover of the remains of deceased must comply with the prohibition of cruel, inhuman or degrading treatment, ensuring respect for the dignity of the deceased and their families," he said in a statement.

Israelis lined the road in the rain near the Gaza border to pay their respects as the convoy carrying the coffins drove by.

"We stand here together, with a broken heart, the sky is also crying with us and we pray to see better days," said one woman, who gave her name only as Efrat.

In Tel Aviv, people gathered, some weeping, in a public square, opposite Israel's defense headquarters, at what has come to be known as Hostages Square.

"Agony. Pain. There are no words. Our hearts — the hearts of an entire nation — lie in tatters," said President Isaac Herzog.

Netanyahu vowed to eliminate Hamas in a recorded address released after the remains of the hostages were handed over, saying "the four coffins" obliged Israel to ensure "more than ever" that there was no repeat of the October 7 attack.

"Our loved ones' blood is shouting at us from the soil and is obliging us to settle the score with the despicable murderers, and we will," he said.

Israeli officials have repeatedly asserted that Hamas would be destroyed over the course of the 16-month-old conflict, and that the roughly 250 hostages abducted during the October 2023 Hamas-led attack on Israel would be returned home.

During Thursday's handover, one militant stood beside a poster of a man standing over coffins wrapped in Israeli flags. It read "The Return of the War = The Return of your Prisoners in Coffins".

The purported remains of the boys, their mother Shiri Bibas and a fourth hostage, Oded Lifshitz, were handed over under the Gaza ceasefire agreement reached last month with the backing of the United States and the mediation of Qatar and Egypt.

Kfir Bibas was nine months old when the Bibas family, including their father Yarden, was abducted at Kibbutz Nir Oz, one of a string of communities near Gaza that were overrun by Hamas-led attackers from Gaza.

Hamas said in November 2023 that the boys and their mother had been killed in an Israeli airstrike but their deaths were never confirmed by Israeli authorities.

"Shiri and the kids became a symbol," said Yiftach Cohen, of the Nir Oz kibbutz, which lost around a quarter of its residents, either killed or kidnapped, during the assault.

Yarden Bibas was returned in an exchange for prisoners this month.

Some of those Israelis killed on October 7 were known peace activists.

Lifshitz was 83 when he was abducted from Nir Oz, the kibbutz he helped found. His wife, Yocheved, 85 at the time, was seized with him and released two weeks later, along with another woman.

He was a former journalist and in an op-ed in left-leaning Haaretz in January 2019, he listed what he said were Netanyahu's policy failures, including his rejection of the two-state solution with the Palestinians and a 2011 deal that exchanged more than 1,000 Palestinian prisoners - including Yahya Sinwar, the mastermind of the October 7 attack - for one abducted Israeli soldier.

The handover marks the first return of dead bodies during the current agreement.

Lifshitz's family later said in a statement that they had been informed that his body had been formally identified.

"Our family's healing process will begin now and will not end until the last hostage is returned," the family said.

The prime minister's office said that Lifshitz was murdered in captivity by Islamic Jihad, another militant group in Gaza.

Chen Kugel, the head of the Israel National Center of Forensic Medicine, later said in a televised statement that Lifshitz had been murdered more than a year ago.

Neither Kugel nor the prime minister's said how the cause of death had been determined.

The Hamas-led attack into Israel killed some 1,200 people, according to Israeli tallies, with 251 kidnapped. Israel's subsequent military campaign has killed some 48,000 people, Palestinian health authorities say, and left densely populated Gaza in ruins.

Thursday's handover of bodies will be followed by the return of six living hostages on Saturday, in exchange for hundreds more Palestinians, expected to be women and minors detained by Israeli forces in Gaza during the war.

Negotiations for a second phase, expected to cover the return of around 60 remaining hostages, less than half of whom are believed to be alive, and a full withdrawal of Israeli troops from the Gaza Strip to allow an end to the war, are expected to begin in the coming days.