Israeli Prime Minister Benjamin Netanyahu said late Sunday that his country was "determined to stand against" Iran and its allied armed groups "on all fronts".
As its war against Iran-backed Hamas in Gaza nears its 11th month, Israel has been bracing for retaliation from the Tehran-aligned "axis of resistance" for the assassinations of two senior figures.
Palestinian armed group Hamas's political leader Ismail Haniyeh was killed in Tehran on Wednesday, in an attack blamed on Israel which has not directly commented on it, hours after an Israeli strike on Beirut left Hezbollah military chief Fuad Shukr dead.
Tehran said Monday that "no one has the right to doubt Iran's legal right to punish the Zionist regime" for Haniyeh's killing.
US Secretary of State Antony Blinken told his counterparts from the G7 nations in a conference call on Sunday that any attack, which he expected to be a joint undertaking between Hezbollah and Iran, could happen within 24 to 48 hours, as early as Monday, US news site Axios reported.
Blinken asked his counterparts to place diplomatic pressure on Tehran, Hezbollah and Israel to "maintain maximum restraint", it added.
The United Nations' rights chief Volker Turk called on "all parties, along with those states with influence, to act urgently to de-escalate what has become a very precarious situation".
Israel's military spokesman Rear Admiral Daniel Hagari said on Sunday night that "as of now there is no change" in its policy for protecting civilians.
'Path of dialogue'
Experts and diplomats fear that the expected attack on Israel could rapidly spiral into a regional war.
Turkey on Monday joined multiple Western and other nations calling on their citizens to leave Lebanon, where Hezbollah is based.
Numerous airlines have suspended flights to the country or limited them to daylight hours.
Italian Foreign Minister Antonio Tajani, whose country currently holds the rotating G7 presidency, said in a statement: "Together with our partners, we have expressed strong concern about recent events that threaten to determine a regionalisation of the crisis, starting from Lebanon".
"We call on the parties involved to desist from any initiative that could hinder the path of dialogue and moderation," he added.
On Sunday, Jordan's Foreign Minister Ayman Safadi made a rare trip to the Iranian capital during which he delivered a message from King Abdullah II to President Masoud Pezeshkian.
Jordanian "airspace will probably be a theatre for missiles and anti-missile" fire in any direct Iranian-Israeli clashes, but Amman would strongly object to violations of its sovereignty, said political analyst Oraib Rantawi.
"The Iranians must find other ways to spare Jordan this embarrassment," Rantawi, director of the Amman-based Al Quds Center for Political Studies, told AFP.
The Israel-Hamas war in the Gaza Strip, triggered by the Palestinian group's October 7 attack, has already drawn in Iran-backed militants in Syria, Lebanon, Iraq and Yemen.
Cross-border clashes
Even as the region braced for further escalation, Hezbollah and Israel continued their near-daily cross-border exchanges of fire.
The Lebanese health ministry said four people were killed in two separate strikes on the border towns of Mais al-Jabal and Hula, while Hezbollah said it had targeted military sites in northern Israel with "explosive-laden drones".
The cross-border violence since October has killed at least 549 people in Lebanon, mostly fighters but also including at least 116 civilians, according to an AFP tally.
On the Israeli side, including the annexed Golan Heights, 22 soldiers and 25 civilians have been killed, according to army figures.
Analysts have told AFP that a joint but measured action from Iran and its allies was likely, while Tehran said it expects Hezbollah to hit deeper inside Israel and no longer be confined to military targets.
Israel's ally the United States said it was moving additional warships and fighter jets to the region.
US President Joe Biden is scheduled to meet with his national security team later Monday "to discuss developments in the Middle East", the White House said.
Rockets
The Hamas attack on southern Israel that sparked the Gaza war resulted in the deaths of 1,197 people, mostly civilians, according to an AFP tally based on Israeli official figures.
Israel's retaliatory campaign has killed at least 39,623 people in Gaza, according to the Hamas-run territory's health ministry, which does not give details of civilian and militant deaths.
The toll includes 40 deaths in the past 24 hours, the ministry said Monday.
The Israeli military said around 15 rockets had crossed from the southern Gaza Strip into Israel on Monday, with medics saying they were treating an injured man.
Months of talks, mediated by Qatar, Egypt and the United States, aimed at a ceasefire and a hostage-release deal have repeatedly stalled.
Hamas officials but also some analysts as well as protesters in Israel have accused Netanyahu of prolonging the war to safeguard his hard-right ruling coalition.
The killing of Haniyeh, who was Hamas's lead negotiator in truce talks, "does not suggest Israel is sincerely interested in a ceasefire", said Middle East expert Andreas Krieg.
