President Joe Biden said the US expects Iran to launch an attack against Israel “sooner rather than later” but warned Tehran against pressing ahead.

Such an attack would greatly inflame tensions in the Middle East, undermining efforts by Washington and its allies to prevent Israel’s war on Gaza from spilling into a wider regional conflict. The Israeli offensive in Gaza, which started in October, has killed more than 33,000 people so far and turned the Palestinian enclave into a wasteland of rubble.

Asked by reporters about his message to Iran, Mr Biden simply said: "Don’t.”

"We are devoted to the defence of Israel. We will support Israel. We will help defend Israel and Iran will not succeed," he added.

Israel is braced for an attack in retaliation for last week’s strike on the Iranian consulate in Damascus as Western capitals fear that an already volatile situation in the region could quickly spiral out of control.

Tel Aviv has said it is ready to defend against any reprisals over the 1 April strike that killed Brigadier General Mohammad Reza Zahedi and six other Iranian officers.

Iran believes the attack was carried out by Israel, though Tel Aviv has not spoken about it. Tehran has vowed to respond.

Mr Biden said he would not divulge confidential information but added that his expectation was that an attack by Iran could come "sooner rather than later”.

The comments came as Iran’s supreme leader, Ayatollah Ali Khamenei, said Israel "must be punished and shall be" for an operation he said was equivalent to an attack on Iranian soil.

John Kirby, Biden’s national security communications adviser, said the US is looking at its own force posture in the region in light of Tehran’s threat and is watching the situation very closely.

The US has rushed warships into position to protect Israel and American forces in the region, hoping to head off a direct attack from Iran on Israel that could come as soon as Friday or Saturday, the Wall Street Journal reported on Friday.

The attack could involve up to 100 drones and missiles, CBS reported quoting two unnamed US officials.

The conflict could also involve Iranian proxies, according to unnamed sources in the Biden administration and intelligence officers quoted by CNN. They added, however, that militia groups aligned with Iran did not appear poised to attack US soldiers or assets in the region as Tehran was wary of a dramatic escalation of the conflict.

The moves by the US that are part of an effort to avoid a wider conflict in the Middle East came after a warning from a person familiar with the matter about the timing and location of the potential Iranian attack, according to the Wall Street Journal.

However, a person briefed by the Iranian leadership said that while plans to attack were being discussed, no final decision had been made, the Journal reported.

“We are certainly mindful of a very public and what we consider to be a very credible threat made by Iran in terms of potential attacks on Israel, and that we are in constant communication with our Israeli counterparts about making sure that they can defend themselves against those kinds of attacks,” Mr Kirby said.

Meanwhile, US secretary of state Antony Blinken spoke with the foreign ministers of Turkey, China and Saudi Arabia, urging them to put diplomatic pressure on Iran against escalating the conflict, said State Department spokesperson Matthew Miller.

Mr Blinken also “spoke with Egyptian foreign minister Shoukry about the risk of escalation by Iran”, the spokesperson posted on X. “They also discussed ongoing efforts to achieve a ceasefire, the release of hostages, increased humanitarian aid to Gaza, and protection of civilians.”

India, France, Poland and Russia have issued pleas to their citizens not to travel to the region while the UK and Germany have warned Iran not to escalate tensions.

India advised its citizens against travelling to Iran and Israel until further notice in view of the “prevailing situation in the region”.

India’s foreign ministry said its citizens in the two countries should observe “utmost precautions about their safety and restrict their movements to the minimum”.

The French foreign ministry advised its citizens against travelling to Iran, Lebanon, Israel, and the Palestinian territories.

In a statement on X, the ministry added that relatives of Iran-based diplomats will return to France and that French civil servants are now banned from conducting any missions in Iran, Lebanon, Israel and the Palestinian territories.

On Thursday, the US State Department barred diplomatic staff and their families in Israel from travelling outside of the cities of Tel Aviv, Jerusalem and Be’er Sheva.

The security alert did not mention Iran directly but it followed a warning from Tehran that they would “punish” those it viewed as responsible for the attack in Syria.

“Out of an abundance of caution, US government employees and their family members are restricted from personal travel outside the greater Tel Aviv, Jerusalem and Be’er Sheva areas until further notice,” the US embassy warning read.

“The security environment remains complex and can change quickly depending on the political situation and recent events.”