The Israeli military said Sunday that it had found the bodies of six captives in a tunnel in the southern Gaza Strip, including a US-Israeli and a Russian-Israeli.
Their remains were recovered Saturday "from an underground tunnel in the Rafah area" and returned to Israel where they were formally identified, the military said.
It said the dead captives were Carmel Gat, Eden Yerushalmi, Hersh Goldberg-Polin, Alexander Lobanov, Almog Sarusi and Ori Danino, who were all taken to Gaza during Hamas' October attack on southern Israel.
Gat was taken from the southern Israeli kibbutz community of Beeri, while the remaining five, ages 23 to 32, were abducted from a music festival near the Gaza border.
They were among 251 people taken hostage during the 7 October attack, 97 of whom remain captive in Gaza including 33 the Israeli army says are dead.
Israeli Defence Minister Yoav Gallant said the six captives whose remains had been retrieved were alive when taken captive.
"They were held hostage by Hamas and murdered in cold blood," Gallant said in a statement.
Military spokesman Rear Admiral Daniel Hagari said "they were brutally murdered by Hamas [fighters] shortly before we reached them".
Senior Hamas official Izzat al-Risheq said the six captives were killed in Israeli air strikes, blaming the United States for its "bias, support and partnership" in the ongoing war on the enclave which has plunged it into a deep humanitarian crisis.
Israeli Prime Minister Benjamin Netanyahu, under pressure after nearly 11 months of war to reach a deal that includes a ceasefire and the release of remaining hostages, said Israel would not rest until it caught those responsible.
"Whoever murders hostages - does not want a deal," he said, vowing to "settle the score" with Hamas and saying he will "hunt" them down.
Senior Hamas officials also said that Israel, in its refusal to sign a ceasefire agreement, was to blame for the deaths.
"Netanyahu is responsible for the killing of Israeli prisoners," senior Hamas official Sami Abu Zuhri told news agency Reuters. "The Israelis should choose between Netanyahu and the deal."
Campaign group the Hostages and Missing Families Forum also insisted that the only way to bring the remaining hostages to Israel was a negotiated deal.
"A deal for the return of the hostages has been on the table for over two months," it said in a statement.
"Were it not for the delays, sabotage, and excuses those whose deaths we learned about this morning would likely still be alive."
The Hostage Families Forum called on Netanyahu to take responsibility and explain what was holding up an agreement.
"They were all murdered in the last few days, after surviving almost 11 months of abuse, torture, and starvation in Hamas captivity. The delay in signing the deal has led to their deaths and those of many other hostages," it said.
Months long talks for a ceasefire and to secure the release of hostages have so far failed to have a positive outcome, with both Israel and Hamas blaming each other for the delays.
The 7 October Hamas attack resulted in the deaths of 1,205 people, mostly civilians, according to Israeli figures.
Israel's offensive has so far killed at least 40,691 people in Gaza, according to the territory's health ministry. The UN rights office says most of the dead are women and children.
'Devastated and outraged'
US President Joe Biden, who has closely followed the fate of the hostages, said the six included Israeli American Goldberg-Polin and that he was "devastated and outraged".
"Hamas leaders will pay for these crimes. And we will keep working around the clock for a deal to secure the release of the remaining hostages," he said in a statement.
Goldberg-Polin, 23, was captured at a music festival near the Gaza border and appeared in a video released by Hamas in late April.
Earlier, speaking to reporters in Rehoboth Beach, Delaware, Biden said he was "still optimistic" about a ceasefire deal to stop the conflict, adding that "people are continuing to meet."
The two sides have agreed to pause fighting for at least eight-hours daily from Sunday to Tuesday to allow the UN World Health Organization (WHO) and Palestinian medics to begin to vaccinate 640,000 children in Gaza.
The campaign comes after the WHO confirmed last month that a baby was partially paralysed by the type 2 polio virus, the first such case in the territory in 25 years.