As the United States presses for a cease-fire between Israel and Hamas, the militant group’s decision will largely hinge on its new de facto leader in Gaza.

The commander, Izz al-Din al-Haddad, took over the military wing in Gaza after Israeli forces killed Muhammad Sinwar, according to a senior Middle Eastern intelligence official and three Israeli defense officials who spoke on condition of anonymity to divulge sensitive details. On Thursday, Brig. Gen. Effie Defrin, the Israeli military’s spokesman, said that Mr. al-Haddad was Hamas’s new leader.

Mr. al-Haddad, who is in his mid-50s, helped plan the Hamas-led attack on Oct. 7, 2023, the officials said.

He is believed to be in firm opposition to Israeli efforts to dislodge Hamas from power, suggesting that he could block any push to release all remaining hostages before a total end to the war in Gaza and a withdrawal of Israeli troops.

“He has the same red lines as the people before him,” said Michael Milshtein, a former Israeli military intelligence officer specializing in Palestinian affairs.