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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 on Gaza ground on.

Days after Israel strengthened its air defences 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 Middle East tensions have soared.

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

Israel's 1 April attack destroyed Iran's consulate building in Damascus and killed seven Revolutionary Guards, including two generals.

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

Israeli Foreign Minister Israel Katz swiftly replied on social media platform 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".

US Central Command chief Michael Kurilla was in Israel on Thursday to discuss the situation with Defence Minister Yoav Gallant, the Pentagon said.

"We warned Iran," White House Press Secretary Karine Jean-Pierre told a briefing without elaborating.

During a visit to an airbase in central Israel, Netanyahu spoke of "challenging times" on multiple fronts.

"We are in the middle of the war in Gaza which continues in full force… but we are also preparing for scenarios of challenges from other arenas," he said in comments released by his office.

Moscow called on both Iran and Israel to exercise restraint.

German Foreign Minister Annalena Baerbock urged "maximum restraint", and Lufthansa said it had extended a temporary suspension of Iran flights until Saturday.

Iranian Foreign Minister Hossein Amirabdollahian said he had received phone calls Thursday from Baerbock as well as her British and Australian counterparts.

In a post on X, he said he had told them that "when the Zionist regime [Israel] breaches the immunity of diplomatic persons and places" and the UN Security Council fails to condemn it, "legitimate defence… is a necessity".

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

Regional tensions have been stoked by Israel's war on Gaza, which has killed at least 33,545 people in Gaza, mostly women and children, according to the enclave's health ministry.

A tightened 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.

A brutal Israeli military campaign has devastated the strip's infrastructure. Ambulances, hospitals, and residential buildings have been attacked.

The International Court of Justice found in January that Israel war violating the UN Genocide Convention in Gaza.