Washington DC - President Joe Biden said on Friday he expects Iran to launch an attack against Israel "sooner than later" as Middle East tensions soar.

Biden said he did not want to elaborate on what classified intelligence Washington had about Tehran's plans.

"We are devoted to the defense of Israel. We will support Israel. We will help defend Israel. And Iran will not succeed," he said.

When asked by a journalist what his message to Tehran would be, Biden replied: "Don't."

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In view of Iran's possible imminent retaliatory strike against Israel, the US earlier announced that it was increasing its military presence in the Middle East.

"We are moving additional assets to the region to bolster regional deterrence efforts and increase force protection for US forces," a US defense official told dpa on Friday, speaking on condition of anonymity.

With the world on high alert, countries including France and India issued travel advisories warning their citizens to avoid Israel and Iran. The US embassy in Israel also issued a security warning for its employees in light of the situation.

Iran's threats towards Israel have increased following an unprecedented airstrike on the Iranian embassy compound in Syria on April 1 which killed several military generals, including Brigadier General Mohammad Reza Zahedi, who was a senior leader in Iran's elite Quds Force, as well as senior advisor Sayyed Razi Mousavi.

Iran's Supreme Leader Ayatollah Ali Khamenei said the airstrike, which Israel has not officially claimed responsibility for, was just like an attack on Iranian territory.

During a prayer ceremony to mark the end of the Muslim holy month of Ramadan on Wednesday, he said Israel had made a mistake and "must and will be punished."

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It would be "imprudent and irresponsible if we weren't also taking a look at our own force posture," US National Security Council spokesman John Kirby said in Washington on Friday. It was a matter of protecting US personnel and facilities in the region, he said.

"We still deem the potential threat by Iran here to be real, to be viable, certainly credible, and we're watching it as closely as we can," he told reporters.

Kirby did not comment on reports that an attack was imminent. "I've seen reporting out there and I can't speak to what the intelligence picture tells us about the size of what that attack might look like," he said.

Israel's devastating assault on Gaza, where it has killed close to 34,000 people, has been accompanied by constant attacks on Lebanon and Syria that threaten to spark a full-blown, region-wide war.