Beirut. Hamas on Tuesday named Yahya Sinwar, its top official in Gaza who masterminded the Oct. 7 attacks in Israel, as its new leader in a dramatic sign of the power of the Palestinian militant group's hardline wing after his predecessor was killed in a presumed Israeli strike in Iran.

The selection of Sinwar, a secretive figure close to Iran who worked for years to build up Hamas' military strength, was a defiant signal that the group is prepared to keep fighting after 10 months of destruction from Israel's campaign in Gaza and after the assassination of Sinwar's predecessor, Ismail Haniyeh.

It is also likely to provoke Israel, which has put him at the top of its kill list after the Oct. 7 attack, in which militants killed 1,200 people in southern Israel and took about 250 as hostages.

The announcement comes at a volatile moment. Fears are high of an escalation into a wider regional war, with Iran vowing revenge against Israel over Haniyeh's killing and Lebanon's Hezbollah threatening to retaliate over Israel's killing of one of its top commanders in an airstrike in Beirut last week. American, Egyptian and Qatari mediators are trying to salvage negotiations over a cease-fire and hostage release deal in Gaza, shaken by Haniyeh's killing.

Hamas said in a statement it named Sinwar as the new head of its political bureau to replace Haniyeh, who was killed in a blast that Iran and Hamas blamed on Israel. Israel has not confirmed or denied responsibility. Also last week, Israel said it had confirmed the death of the head of Hamas’ military wing, Mohammed Deif, in a July airstrike in Gaza. Hamas has not confirmed his death.

In reaction to the appointment, Israeli military spokesman Rear Adm. Daniel Hagari told Saudi-owned Al-Arabiya television, “There is only one place for Yahya Sinwar, and it is beside Mohammed Deif and the rest of the October 7th terrorists. That is the only place we’re preparing and intending for him.”

Israel's killings of multiple senior officials in Hamas over recent months left Sinwar as the most prominent figure in the group. His selection signals that the leadership on the ground in Gaza -- particularly the armed wing known as the Qassam Brigades -- has taken over from the leadership in exile, which has traditionally maintained the position of the overall leadership to navigate relations with foreign allies and diplomacy.

Haniyeh, who had lived in self-imposed exile in Qatar since 2019, had played a direct role in negotiations over a cease-fire in Gaza through US, Qatari and Egyptian negotiators -- though he and other Hamas officials always ran proposals and positions by Sinwar.

Speaking to Al-Jazeera television after the announcement, Hamas spokesman Osama Hamdan said Sinwar would continue the cease-fire negotiations.

“The problem in negotiations is not the change in Hamas,” he said, blaming Israel and its ally the United States for the failure to seal a deal.

But he said Sinwar's selection was a sign the group's will had not been broken. Hamas “remains steadfast in the battlefield and in politics," he said. "The person leading today is the one who led the fighting for more than 305 days and is still steadfast in the field.”

Hamas' allies Iran and Hezbollah issued statements praising Sinwar's appointment.

Hamas' representative in Iran, Khaled Kaddoumi, called Sinwar a “consensus choice” popular among all factions and involved in the group’s decision-making throughout, including in negotiations. In a voice message to the AP, he said Sinwar knows the political aspirations of the Palestinians for a state and the return of refugees but he is also a “fierce fighter on the battlefield.”

Mediators have been struggling to push through a US-backed outline for a deal, but talks have hit obstacles, particularly over its centerpiece terms -- a release of all of Hamas' remaining hostages in return for an end to the war and a full withdrawal of Israeli troops from Gaza.

Hamas has demanded guarantees from mediators that an initial cease-fire will continue until terms for that exchange are worked out. Israeli leaders have threatened to resume fighting to eliminate Hamas after an initial partial hostage release.

In Washington, US Secretary of State Antony Blinken said Sinwar “has been and remains the primary decider when it comes to concluding the cease-fire.”

He said Sinwar must “decide whether to move forward with a cease-fire that manifestly will help so many Palestinians in desperate need, women, children, men who are caught in a crossfire … It really is on him.”

As Hamas' leader inside Gaza since 2017, Sinwar rarely appeared in public but kept an iron grip on Hamas' rule over the territory. Close to Deif and Qassam Brigades, he worked to build up the group's military capabilities.

In one of his few appearances, Sinwar ended a public speech in Gaza by inviting Israel to assassinate him, proclaiming, “I will walk back home after this meeting.” He then did so, shaking hands and taking selfies with people in the streets.

He has been in deep hiding since the Oct. 7 attacks, which triggered Israel's campaign of bombardment and offensives aimed at destroying Hamas. The death toll among Palestinians is now nearing 40,000, most of the population of 2.3 million has been driven from their homes, and large swaths of Gaza's towns and cities have been destroyed.

In May, prosecutors at the International Criminal Court sought an arrest warrant against Sinwar on charges of war crimes over the Oct. 7 attack, as well as against Israeli Prime Minister Benjamin Netanyahu and Israel's defense minister for war crimes.