Washington DC - Iran is preparing an imminent ballistic missile attack against Israel, the US said Tuesday, warning that any such assault would have "severe" consequences for Tehran.

The warning came as Israel said it had launched a ground offensive in Lebanon to target the Iran-backed militia Hezbollah, whose top leader, Hassan Nasrallah, was killed in an Israeli air strike last week.

"The United States has indications that Iran is preparing to imminently launch a ballistic missile attack against Israel," a senior White House official told AFP on condition of anonymity.

"We are actively supporting defensive preparations to defend Israel against this attack."

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Following the warning, the Israeli military said it had not detected the same threat so far.

"At this moment, we do not detect any aerial threat launched from Iran," said military spokesman Rear Admiral Daniel Hagari.

"We are on high alert alongside our partners in the United States, our ally, and are closely monitoring developments in Iran."

Hagari said the country's air defense systems were fully prepared while combat aircraft continued to patrol its skies.

"We are at peak readiness on both offense and defense," he said. "We have dealt with this type of threat before, and we will deal with it now as well."

The US official added: "A direct military attack from Iran against Israel will carry severe consequences for Iran."

The US and other Western allies stepped in to help defend Israel against a combined Iranian missile and drone attack in April, which Tehran launched in retaliation for an Israeli strike on the Iranian consulate in Damascus.

Iran has said that Nasrallah's killing would bring about Israel's "destruction," though the foreign ministry said Monday that Tehran would not deploy soldiers to confront Israel.

Israeli Prime Minister Benjamin Netanyahu meanwhile issued a stark warning to the people of Iran on Monday, saying there was "nowhere in the Middle East Israel cannot reach."

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Netanyahu also said that a future "when Iran is finally free" would "come a lot sooner than people think."

An attack on Israel by Iran would gravely compound fears of a wider regional conflict that the United States and other world powers have said they want to avoid in the Middle East.

The United States has cautiously backed Israel's move to dismantle Hezbollah's ability to attack northern Israel, even as President Joe Biden has called for a ceasefire.

US Secretary of State Antony Blinken on Tuesday said that Washington was "tracking events in the Middle East very closely."

"The United States is committed to Israel's defense," Blinken said Tuesday morning while meeting his Moroccan counterpart Nasser Bourita at the State Department.

Washington said Monday that it was boosting its forces in the Middle East by a "few thousand" troops, by bringing in new units while extending others that are already there. It was also deploying more fighter jets, the Pentagon said.

US Defense Secretary Lloyd Austin late Monday offered support to his Israeli counterpart Yoav Gallant for "dismantling attack infrastructure" belonging to Hezbollah along the border with Lebanon.

Hezbollah began low-intensity strikes on Israeli troops a day after its Palestinian ally Hamas, which is also backed by Iran, staged its unprecedented attack on Israel on October 7, which triggered Israel's devastating assault on the Gaza Strip.

Cross-border fire from Israel and Hezbollah continued throughout Israel's war in Gaza, and late on Monday, Israel announced it had launched ground raids in southern Lebanon targeting Hezbollah positions.