Israel was on alert Thursday after its arch-foe Iran threatened reprisals over a strike in Syria this month that killed two Iranian generals and as the war against Hamas ground on in Gaza.

Days after Israel strengthened its air defenses and paused leave for combat units, the United States also warned of the risk of an attack by Iran or its allied groups at a time when Middle East tensions have soared.

Iran is "threatening to launch a significant attack on Israel," US President Joe Biden said Wednesday, pledging "ironclad" support for its top regional ally despite diplomatic tensions over Israel's military conduct in Gaza.

Israel was widely blamed for an April 1 attack that destroyed Iran's consulate building in Damascus and killed seven members of the Islamic Revolutionary Guard Corps (IRGC), including two generals.

Iran's supreme leader, Ayatollah Ali Khamenei, on Wednesday, warned that Israel "must be punished and will be punished", days after one of his advisors had said that Israeli embassies are "no longer safe".

Israeli Foreign Minister Israel Katz swiftly replied to Khamenei on social media site X that "if Iran attacks from its territory, Israel will respond and attack Iran".

Biden said he had told Israeli Prime Minister Netanyahu that "our commitment to Israel's security against these threats from Iran and its proxies is ironclad".

"Let me say it again -- ironclad. We're going to do all we can to protect Israel's security."

The Axios news site reported that US Central Command chief Michael Kurilla was set to visit Israel to discuss the situation with Defence Minister Yoav Gallant.

Moscow called on both Iran and Israel to exercise restraint, after earlier warning Russian citizens to refrain from traveling to Israel, Lebanon, and the Palestinian territories.

"It is very important for everybody to exercise restraint in order not to destabilize the region, which is already not gifted with stability or predictability," Kremlin spokesman Dmitry Peskov told reporters.

German airline Lufthansa said it had suspended flights to and from Tehran, probably until Thursday, "due to the current situation in the Middle East".

Israel and the United States have long faced off against Iran and the so-called "Axis of Resistance" coalition of militant groups based in Lebanon, Iraq, Syria, and Yemen.

The Syria strike killed Iranian IRGC Brigadier General Mohammad Reza Zahedi, who led the foreign operations wing Quds Force in Syria and Lebanon.

 'Panic among children' 

 Regional tensions have been stoked by the Gaza war which broke out after Hamas launched their October 7 attack against Israel, which left 1,170 people dead, mostly civilians, according to Israeli figures.

Palestinian militants also took about 250 hostages, 129 of whom remain in Gaza, including 34 the Israeli army says are dead.

Iran has said it had no advance knowledge of the October 7 attack but has hailed the assault against its decades-old enemy.

Israel's retaliatory offensive has killed at least 33,545 people in Gaza, mostly women and children, according to the Hamas-run territory's health ministry.

The Israeli military reported overnight operations in central Gaza which had also involved its navy and air force "to eliminate terrorist operatives".

In Gaza's Nuseirat area, Imad Abu Shawish, 39, said "The situation is dire and still getting worse, bombardment hasn't stopped and is still happening now.

"We hear the sounds of missiles falling near us before they explode, which is causing panic among my children and other family members."

Much of the Palestinian territory has been reduced to a bomb-cratered wasteland of destroyed buildings with yet more bodies feared under the mountains of rubble.

An Israeli siege has deprived Gaza's 2.4 million people of most food, water, fuel, and medicines, the dire shortages only alleviated by sporadic aid deliveries.

Israeli war cabinet member Benny Gantz said on Wednesday that "Hamas is defeated" militarily but pledged to keep fighting "what remains of it" in the years to come.

An Israeli air strike on Wednesday killed three sons of Hamas's Qatar-based leader Ismail Haniyeh.

Haniyeh's brother Nahed told AFP in Gaza that their family is "in the same situation" as other Gazans.

"There is no difference between the sons of leaders and the sons of the people," he said.

Haniyeh insisted that his sons' deaths would not influence Hamas' position in ongoing talks in Cairo on a possible temporary ceasefire and hostage release deal.

Those talks, which started Sunday, have brought no breakthrough on a plan presented by US, Qatari, and Egyptian mediators, which Hamas said it was studying.

A framework plan would halt fighting for six weeks and see the exchange of about 40 hostages for hundreds of Palestinian prisoners, as well as more aid deliveries.

Biden said that "it's now up to Hamas, they need to move on the proposal that's been made".

 'Destabilising Middle East' 

Washington has also ramped up pressure on Netanyahu to agree to a truce, increase aid flows, and abandon plans to invade the territory's far southern city of Rafah.

About 1.5 million civilians are sheltering in Rafah, the last Gazan city yet to face a ground incursion.

Biden labeled Netanyahu's handling of the war a "mistake" in an interview broadcast on Tuesday.

Gallant promised Israel would "flood Gaza with aid", using an Israeli crossing point, streamlined checks, and two new routes organized with Jordan.

He said they expected to hit 500 aid trucks entering Gaza a day, the average pre-war level.

Israel has faced a chorus of international criticism over its handling of the war.

Spain is among several Western nations, including Ireland and Australia, to have suggested they would recognize a Palestinian state shortly as a starting point for wider peace talks.

Spanish Prime Minister Pedro Sanchez warned that Israel's "disproportionate response" in Gaza risked "destabilising the Middle East and, as a consequence, the entire world".