Abu Ubaida blamed the incident on what he described as Israeli “massacres” against Palestinians.
“The enemy government bears full responsibility for these massacres and the resulting reactions that affect the lives of Zionist prisoners,” Abu Ubaida said in a statement posted on Telegram.
He said a committee had been formed to investigate and findings would be announced later, and efforts were under way to save the two wounded hostages.
It’s the first time al-Qassam has said its guards killed hostages.
The group has often attributed previous killings of hostages to Israeli bombardment.
Israeli Army spokesman Avichay Adraee said in Arabic on X: “In the last few minutes, the terrorist Hamas published a written report claiming that in two separate incidents, Hamas activists killed an Israeli captive and wounded two women captives. At this stage there is no intelligence document to confirm or refute Hamas’ allegations. We continue to investigate the credibility of the statement and will provide information where we have it.”
Earlier, the Palestinian group named Gaza leader Yahya Sinwar as its new leader following the assassination of former leader Ismail Haniyeh in Tehran on July 31.
A ceasefire deal would aim to end war in the Gaza Strip and ensure the release of Israeli hostages held in the enclave in return for many Palestinians jailed by Israel.
Hamas wants a deal to end the war while Israeli Prime Minister Benjamin Netanyahu says the war would end only after Hamas is eradicated.