The United States said it would move additional warships and fighter jets to the region as the Iran-aligned "Axis of Resistance" readied its response to the killing of Ismail Haniyeh.
The groups from Lebanon, Yemen, Iraq and Syria have already been drawn into the nearly 10-month war in Gaza between Israel and the Palestinian movement Hamas.
Iran on Saturday said it expects one of those groups, Lebanon's Hezbollah, to hit deeper inside Israel and to no longer be confined to military targets.
With such talk growing, the Pentagon said it was bolstering its military presence in the Middle East to protect US personnel and defend Israel.
An aircraft carrier strike group led by the USS Abraham Lincoln will replace one helmed by the USS Theodore Roosevelt in the region, the Pentagon said.
Defense Secretary Lloyd Austin also ordered additional ballistic missile defence-capable cruisers and destroyers to the Middle East and areas under United States European Command, as well as a new fighter squadron to the Middle East.
On Friday, thousands of people in Qatar attended funeral prayers for Haniyeh, who was buried north of the capital Doha two days after his death.
Iran's Revolutionary Guards on Saturday said he was killed by a "short-range projectile" fired "from outside the accommodation area" where he was staying.
Haniyeh had been in Iran to attend the swearing-in of President Masoud Pezeshkian on Tuesday.
Israel, accused by Hamas, Iran and others of the attack, has not directly commented on it.
Tit-for-tat
The killing of the Qatar-based Haniyeh is among a series of tit-for-tat attacks since April that had already heightened fears of a regional conflagration.
His death came hours after Israel struck south Beirut, killing the Hezbollah military commander Fuad Shukr.
Haniyeh's deputy was killed in south Beirut early this year in a strike which a US defence official said Israel carried out.
In another high-profile killing, Israel's army on Thursday confirmed that an air strike in July killed Hamas military chief Mohammed Deif in Gaza.
Israel "delivered crushing blows to all our enemies", said Prime Minister Benjamin Netanyahu this week.
"The risk that the situation on the ground could deteriorate rapidly is rising," British Foreign Secretary David Lammy said in a statement.
Israeli Defence Minister Yoav Gallant met with his visiting British counterpart John Healey on Friday and called for an international coalition to support "Israel's defence against Iran and its proxies", Gallant's office said.
Israel has vowed to destroy Hamas in retaliation for its October 7 attack which triggered war in Gaza and resulted in the deaths of 1,197 people, mostly civilians, according to an AFP tally based on official Israeli figures.
Militants also seized 251 hostages, 111 of whom are still held captive in Gaza, including 39 the military says are dead.
Israel's retaliatory campaign against Hamas has killed at least 39,550 people in Gaza, according to the health ministry in the Hamas-ruled territory, which does not give details of civilian and militant deaths.
Violence has also surged in the Israeli-occupied West Bank, where on Saturday Palestinian official sources said an Israeli drone strike killed five people in a vehicle.
The military said it struck "five terrorists" on their way to carry out an attack.
Haniyeh played a key role in mediated talks aimed at ending the war in Gaza. His killing raised questions about the continued viability of such negotiations which Qatar, Egypt and the United States have engaged in for months.
Hepatitis A spreading
Hamas officials but also some analysts, and protesters in Israel, have accused Netanyahu of prolonging the war.
Far-right members crucial to Netanyahu's ruling coalition oppose any truce.
The war in Gaza has caused widespread destruction and displaced almost the entire population of the territory where, the UN said on Friday, public health conditions "continue to deteriorate."
It said nearly 40,000 cases of Hepatitis A, spread by contaminated food and water, have been reported since the war began.
Since October Hezbollah has been exchanging near-daily fire with Israeli forces, saying it is targeting military positions over the border in support of Hamas.
The strike on Shukr changed the calculus, Iran's mission to the United Nations said on Saturday.
"We expect... Hezbollah to choose more targets and (strike) deeper in its response," said the mission, quoted by Iran's official IRNA news agency.
"Secondly, that it will not limit its response to military targets."
A Lebanese security source, requesting anonymity as they were not authorised to speak to the media, said a Hezbollah member was killed in an "Israeli drone" strike on a vehicle in south Lebanon on Saturday.
Late on Friday, a source close to Hezbollah said Israel carried out strikes on a convoy of trucks entering Lebanon from Syria.
Flights to Beirut by Air France and low-cost carrier Transavia France will remain suspended until at least Tuesday, but Tel Aviv-bound flights continue as normal, their parent company said on Saturday.
Sweden on Saturday said it was shutting its embassy in Beirut and urged its nationals to leave the country "while they still can."
burs-str-it/dcp
