Israel has confirmed the deaths of six more hostages taken in the 7 October attack by Hamas, saying they had been killed by their captors shortly before their bodies were found on Saturday in a tunnel complex under Gaza.

“According to our initial estimation, they were brutally murdered by Hamas terrorists a short time before we reached them,” a military spokesperson, Rear Adm Daniel Hagari, told reporters in an early morning briefing.

The news of the discovery of the bodies brought calls for a mass protest from a hostage family organisation, which blamed the Israeli prime minister, Benjamin Netanyahu, for failing to agree a hostage-for-peace deal with Hamas that has been under negotiation for several months. The country would “tremble”, the organisation warned.

There was no immediate statement from Netanyahu on Sunday morning but Israel’s president, Isaac Herzog, said the nation would continue the fight against Hamas while putting a priority on rescuing the remaining hostages.

“The blood of our brothers cries out to us,” Herzog said. “Our sisters and brothers are still there enduring hell. The supreme covenant between the state and its citizens is to ensure their safety. We have the sacred and urgent mission to bring them home.”

The discovery of the bodies leaves 101 hostages still unaccounted for in Gaza. The IDF has confirmed 35 of them are known to have died in over 10 months of war in Gaza since the 7 October 2023 attack by Hamas on Israel.

The Israeli military (IDF) first reported on Saturday night that bodies had been found “during combat” but said work was still under way in extracting the remains and then identifying them. Shortly after 7am on Sunday, the military confirmed it had located and recovered the bodies of Carmel Gat, Eden Yerushalmi, Hersh Goldberg-Polin, Alexander Lobanov, Almog Sarusi, and Master Sergeant Ori Danino on Saturday from an underground tunnel in the Rafah area in the Gaza Strip.

“They were all taken hostage on October 7th and were murdered by the Hamas terrorist organisation in the Gaza Strip,” said an IDF statement. “Following an identification procedure carried out by the National Institute of Forensic Medicine, the Israel police, and the IDF military rabbinate, the IDF manpower directorate’s hostage team, which is responsible for accompanying the families of the hostages, notified their families.

“The IDF and ISA [Israel Security Agency, the Shin Bet] send their heartfelt condolences to the families. The IDF and Israeli security forces are operating with all means to bring home all the hostages as fast as possible.”

The defence minister, Yoav Gallant, said they were “murdered in cold blood”.

“I commend the IDF and ISA forces for conducting a complex operation to retrieve the bodies of the hostages for burial in Israel,” Gallant said in a statement.

An organisation representing many relatives of the abductees, the Hostage Families Forum, called for a nationwide protest against the Netanyahu government, which it has long accused of dragging its feet over a hostage deal with Hamas that the US and its regional allies have been trying to broker since the end of May.

“Netanyahu abandoned the abductees. This is now a fact,” the forum said in a statement issued on Saturday night when the first reports emerged of bodies having been found. “Starting tomorrow the country will tremble. We call on the public to prepare to bring the country to a standstill.”

It said in a further statement on Sunday: “These six individuals were taken alive, endured the horrors of captivity, and were then coldly murdered … A deal for the return of the hostages has been on the table for over two months. Were it not for the delays, sabotage, and excuses those whose deaths we learned about this morning would likely still be alive.”

The family of 23-year-old Israeli-American Hersh Goldberg-Polin had confirmed his death a few hours before the IDF named the victims on Sunday.

“With broken hearts, the Goldberg-Polin family is devastated to announce the death of their beloved son and brother, Hersh,” the statement said.