Hamas named the alleged mastermind of the deadliest attack in Israeli history as its new leader, further inflaming regional tensions as the Gaza war entered its 11th month on Wednesday.
Yahya Sinwar's appointment angered Israel at a time when it is bracing for potential Iranian retaliation for the killing of his predecessor Ismail Haniyeh last week in Tehran.
Israeli Foreign Minister Israel Katz said Sinwar's promotion was "yet another compelling reason to swiftly eliminate him and wipe this vile organisation off the face of the earth".
Sinwar -- the Palestinian militant group's leader in Gaza since 2017 -- has not been seen since the October 7 attack, after which Israel called him a "dead man walking".
A senior Hamas official told AFP that 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 more reluctant to agree to a Gaza ceasefire 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 as he "has been and remains the primary decider".
Pope Francis called for "an immediate ceasefire on all fronts".
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 his group and Tehran were "obliged to respond."
Nasrallah said Hezbollah would retaliate "alone or in the context of a unified response from all the axis" of Iran-backed groups in the region, "whatever the consequences".
Minutes before his speech, Israeli jets flew low over the Lebanese capital in a show of force.
The United States, which has sent extra warships and fighter jets to the region, urged both Iran and Israel to avoid an escalation.
President Joe Biden called Jordan's King Abdullah II, whose country helped down Iranian drones and missiles in an attack on Israel in April.
It was followed by a call with the Qatari emir, Sheikh Tamim bin Hamad Al Thani, and another with Egyptian President Abdel Fattah al-Sisi, whose countries have been the key intermediaries seeking a ceasefire in the 10-month Gaza war.
"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.
The Jeddah-based Organisation of Islamic Cooperation is to meet on Wednesday at the request of "Palestine and Iran", to discuss developments in the region, an OIC official said.
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 on a Druze Arab town in the Israeli-annexed Golan Heights that killed 12 children, calling him the "right-hand man" of Nasrallah.
Hezbollah has traded near-daily cross-border fire with Israeli troops throughout the Gaza war.
The group said Tuesday that six of its fighters were killed in Israeli strikes on south Lebanon and that it had launched "dozens of Katyusha rockets" at a military base in the Golan Heights in retaliation.
Regional councils in northern Israel urged residents to stay close to shelters after a barrage of rockets.
Numerous airlines have suspended flights to Lebanon or limited them to daylight hours.
Lebanese national carrier Middle East Airlines has added extra flights for people wanting to leave or return, a company source said.
Germany's Lufthansa said it would avoid Iranian and Iraqi airspace until August 13.
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.
Israel said that its air force had "struck dozens of terror targets throughout the Gaza Strip" over the past day.
