Photo: REUTERS/Mohammed Salem/File photo.

Mr Sinwar, who planned the deadliest attack in Israel in decades, has been holed up in Gaza, defying Israel since the war there began.

In a statement, Hamas said: "The Hamas Islamic Resistance Movement announces that Commander Yahya Sinwar will become the leader of the movement's political bureau, succeeding the martyr Commander Ismail Haniyeh, may Allah bless him."

"This appointment means that Israel will have to negotiate with Mr Sinwar to reach a solution to the war in Gaza," said a diplomat with information about the negotiations brokered by Egypt and Qatar to end the fighting in Gaza and return 115 hostages to their homeland.

"This is a message of perseverance and uncompromising."

Mr Sinwar, who has spent half his life in Israeli prisons, is the most senior Hamas leader to survive the assassination of Mr Haniyeh, an attack that brought the region to the brink of widespread conflict after Iran promised a harsh response.

Israel has not claimed responsibility for the assassination but said it had killed several other senior leaders, including Hamas deputy commander Saleh al-Arouri in Beirut and Mohammad Deif, the group's military chief.

Born in a refugee camp in the southern Gaza city of Khan Younis, Mr Sinwar, 61, was elected Hamas leader in Gaza in 2017, after building a reputation as a heavy-handed user of force and a sworn enemy of Israel.

Israel's chief military spokesman, Rear Admiral Daniel Hagari, accused Mr Sinwar of being behind the October 7 attack and said Israel would continue to hunt him down.

"There is only one place for Yahya Sinwar, and that is next to Mohammad Deif and the other terrorists who took part in the October 7 attack. That is the only place we are planning to take him," he said on Al-Arabiya television, according to a statement from the Israeli military.

Ceasefire negotiations

There are signs that Hamas has rallied around Sinwar, with senior sources within the organization saying that Khaled Meshaal, a former leader once seen as a potential successor to Haniyeh, has backed Sinwar out of “loyalty to Gaza and its people, who are fighting in the Al-Aqsa Flood.”

For Israel, the appointment confirms Hamas as an enemy determined to destroy the country and will likely reinforce Israeli Prime Minister Benjamin Netanyahu's insistence that the Gaza campaign must be carried out to the end.

The White House declined to comment on Sinwar’s appointment, but a person with knowledge of Washington’s internal affairs said the decision suggested Hamas might adopt a harder line in ceasefire negotiations and make efforts to reach a deal more difficult.

However, the person also said that Israel had known that Mr. Sinwar would have the final say on ceasefire agreements even before his appointment, and that this statement only formalized that position.

Ten months have passed since the surprise attack on October 7 by Hamas in Israel, and the response has shaken up the Middle East with the threat of what could quickly become a wider regional conflict.

About 1,200 Israelis were killed and 250 others were taken hostage in the October 7 attack. Israel's response campaign in Gaza has killed nearly 40,000 Palestinians and devastated the densely populated enclave.

Efforts to reach a ceasefire agreement that would bring peace to the people of the enclave and allow hostages to return home have encountered many obstacles as well as mutual condemnation from Hamas and Israel.

Hamas official Osama Hamdan told Al Jazeera that the movement remains committed to reaching an agreement and that the negotiating delegation serving under Haniyeh will continue to work under Sinwar.

However, Hani Al-Masri, a Ramallah-based political analyst, said Sinwar's appointment as the movement's overall leader was a direct challenge to Israel, and sent a message about Hamas' "extremist and defiant approach".

"Since Mr. Sinwar controls the negotiation, he will control the organization."

Nguyen Quang Minh (according to Reuters)