Israel vowed to eliminate new Hamas chief Yahya Sinwar, the alleged mastermind of the October 7 attack, with regional tensions threatening to boil over as the Gaza war entered its 11th month on Wednesday.
The naming of Sinwar to lead the Palestinian militant group came as Israel braced for potential Iranian retaliation over the killing of his predecessor Ismail Haniyeh last week in Tehran.
Speaking at a military base on Wednesday, Prime Minister Benjamin Netanyahu said Israel was "determined" to defend itself.
"We are prepared both defensively and offensively," he told new recruits.
Army chief Lieutenant General Herzi Halevi vowed to "find him (Sinwar), attack him" and force Hamas to find another leader.
Sinwar -- Hamas's leader in Gaza since 2017 -- has not been seen since the October 7 attack, the deadliest in Israel's history.
A senior Hamas official told AFP the selection of Sinwar sent a message that the organisation "continues its path of resistance".
Hamas's Lebanese ally Hezbollah congratulated Sinwar and said the appointment affirms "the enemy... has failed to achieve its objectives" by killing Hamas leaders and officials.
Analysts believe Sinwar has been both more reluctant to agree to a Gaza ceasefire and closer to Tehran than Haniyeh, who lived in Qatar.
"If a ceasefire deal seemed unlikely upon Haniyeh's death, it is even less likely under Sinwar," according to Rita Katz, executive director of the SITE Intelligence Group.
"The group will only lean further into its hardline militant strategy of recent years," she added.
US Secretary of State Antony Blinken told reporters that it was up to Sinwar to help achieve a ceasefire, saying he "has been and remains the primary decider".
Civilians in both Israel and Gaza met Sinwar's appointment with unease.
Mohammad al-Sharif, a displaced Gazan, told AFP: "He is a fighter. How will negotiations take place?"
In Tel Aviv, logistics company manager Hanan, who did not want to give his second name, said Sinwar's appointment meant Hamas "did not see fit to look for someone less militant, someone with a less murderous approach".
- Hezbollah vows response -
Iran-backed Hezbollah has also pledged to avenge the deaths of Haniyeh and its own military commander Fuad Shukr in an Israeli strike in Beirut hours earlier.
In a televised address to mark one week since Shukr's death, Hezbollah leader Hassan Nasrallah said Tuesday his group would retaliate "alone or in the context of a unified response from all the axis" of Iran-backed groups in the region.
The United States, which has sent extra warships and jets to the region, urged both Iran and Israel to avoid an escalation.
President Joe Biden had calls with Jordan's King Abdullah II, the Qatari emir, Sheikh Tamim bin Hamad Al Thani, and Egyptian President Abdel Fattah al-Sisi on Tuesday.
"No one should escalate this conflict. We've been engaged in intense diplomacy with allies and partners, communicating that message directly to Iran. We communicated that message directly to Israel," Blinken told reporters.
France's President Emmanuel Macron told Netanyahu on Wednesday to "avoid a cycle of reprisals", after earlier delivering the same message to his Iranian counterpart Masoud Pezeshkian, the French presidency said.
Pezeshkian told Macron in a separate telephone call that the West "should immediately stop selling arms and supporting" Israel if it wants to prevent war, his office said.
The Jeddah-based Organisation of Islamic Cooperation on Wednesday said Israel was "fully responsible" for the "heinous attack" that killed Haniyeh, which it described as "a serious infringement" of Iran's sovereignty.
Gambian Foreign Minister Mamadou Tangara, whose country currently chairs the bloc, said the killing risked "leading to a wider conflict that could involve the entire region".
Israel has not commented on Haniyeh's killing but confirmed it had carried out the strike on Shukr.
It held the Hezbollah commander responsible for a rocket attack in the Israeli-annexed Golan Heights that killed 12 children.
- Flights cancelled -
Hezbollah has traded near-daily cross-border fire with Israeli troops throughout the Gaza war.
On Wednesday, a Lebanese security source said that a Hezbollah fighter and a civilian were killed in an Israeli strike near Jouaiyya close to the border. The Israeli military said it had eliminated a Hamas commander in the area.
Numerous airlines have suspended flights to Lebanon or limited them to daylight hours, while Egypt said Iran had warned civilian airlines to steer clear of its airspace as it will be conducting military exercises overnight.
The United Nations said it was "temporarily" reducing the presence of UN staff family members in Lebanon, although it was not moving its staff.
The Israel-Hamas war in the Gaza Strip, triggered by the Palestinian group's unprecedented October 7 attack on Israel, has already drawn in Iran-backed militants in Syria, Lebanon, Iraq and Yemen.
The Hamas attack resulted in the deaths of 1,198 people, mostly civilians, according to an AFP tally based on Israeli official figures.
Palestinian militants seized 251 hostages, 111 of whom are still held in Gaza, including 39 the Israeli military says are dead.
Israel's retaliatory military campaign in Gaza has killed at least 39,677 people, according to the Hamas-run territory's health ministry, which does not give details of civilian and militant deaths.
The toll included two dozen deaths in the past 24 hours, according to ministry figures.
Israel said its air force had "struck dozens of terror targets throughout the Gaza Strip" over the past day.