A senior Hamas official, Izzat Alishk, claimed Israel was responsible for the deaths because of its refusal to sign the ceasefire agreement.
Israeli Prime Minister Benjamin Netanyahu said Israel would not rest until those who killed them were brought to justice.
He said in a statement that the government was committed to reaching a deal to free those still in captivity and protect the country's security.
“Anyone who kills hostages doesn't want to make a deal,” he added.
A group representing families of those held hostage in the Gaza Strip called on Netanyahu to “address the nation and take responsibility for abandoning the hostages.”
The Hostage Families Forum said all six prisoners had been “abused, tortured and starved in Hamas captivity for nearly 11 months before being killed in the past few days.”
“The delay in signing the deal led to their deaths and those of many other hostages,” they added in a statement.
The group also announced plans to “stop the country” on Sunday, asking Israelis to take part in protests in Jerusalem, Tel Aviv and other parts of Israel demanding a hostage exchange deal.
A Bedouin Arab who was rescued by Israeli forces in Gaza last week has urged Israel to negotiate with Hamas to release all remaining hostages.
“Happiness is not complete as long as there are detainees on both sides,” Elkadi said after returning to his village in southern Israel on Wednesday.
The IDF said in a statement announcing the deaths Sunday morning that the bodies had been “returned to Israeli territory.”
“They were all taken hostage on October 7, 2023 and killed by the Hamas terrorist organization in the Gaza Strip.”
The statement added that their families had already been notified.
Israeli President Isaac Herzog said the news of their murders “shattered the hearts of the entire nation.”
He added: “On behalf of the State of Israel, I offer my heartfelt condolences to the families and apologize for not being able to bring them home safely.”
