US President Joe Biden on Wednesday promised "ironclad" support for Israel as Iran threatens reprisals over a strike that leveled an Iranian consulate building in Damascus and killed two generals.

Biden's promise comes despite his public criticism of Prime Minister Benjamin Netanyahu over the toll on civilians in Israel's campaign against Hamas, especially after a strike killed seven aid workers.

Iran is "threatening to launch a significant attack on Israel," Biden told a news conference.

"As I told Prime Minister Netanyahu, our commitment to Israel's security against these threats from Iran and its proxies is ironclad," Biden said.

"Let me say it again -- ironclad. We're going to do all we can to protect Israel's security," said Biden, who was speaking next to Japanese Prime Minister Fumio Kishida.

US officials have voiced alarm over the prospect of an imminent strike against Israeli interests after Israel on April 1 destroyed the consulate building, killing seven members of Iran's elite Revolutionary Guards, including two generals.

Amid the heightened risks, German airline Lufthansa on Wednesday announced it had suspended flights to and from Tehran, probably until Thursday, saying it was "due to the current situation in the Middle East."

Iran's Supreme Leader Ayatollah Ali Khamenei warned in a speech Wednesday that the "evil regime" of Israel "must be punished and will be punished."

Israeli Foreign Minister Israel Katz swiftly riposted, in a Persian-language statement on social media site X: "If Iran attacks from its territory, Israel will respond and attack Iran."

Iran's clerical state supports Hamas, which on October 7 launched the deadliest attack against Israel in the country's history, triggering a relentless six-month Israeli military operation inside the Hamas-ruled Gaza Strip.

The United States since the start of the war has sought to prevent it from spreading, including to Lebanon, where Iran supports the Shiite militant movement Hezbollah.

The United States has been tight-lipped in its public reaction to the April 1 strike, saying it has not determined whether Israel struck a diplomatic facility, which would breach international agreements on the inviolability of embassies and consulates.