IRAN’s Supreme Leader has warned of “severe revenge” after the country’s most powerful military leader was killed in a US air strike ordered by Donald Trump.

General Qasem Soleimani was targeted by a drone early yesterday morning as he and Iranian backed militia leaders left Baghdad airport in neighbouring Iraq.

Announcing three days of mourning, Ayatollah Khamenei said “severe revenge awaits the criminals” behind the strike.

After ordering the attack from his Mar-a-Lago resort in Florida, President Trump claimed Soleimani had been “directly and indirectly responsible for the deaths of millions of people”.

He also blamed him for killing and wounding thousands of Americans and plotting the deaths of many more, and tweeted an image of the American flag after the death was confirmed.

At a press conference tonight, the president added:"We can take comfort knowing that his reign of terror is over.

"We do not take action to start a war. I am ready and prepared to take whatever action is necessary. And that particularly refers to Iran."

The Pentagon said the US military had taken “decisive defensive action to protect US personnel abroad by killing Qasem Soleimani”, who it said had been “developing plans to attack American diplomats and service members in Iraq and throughout the region”.

The US military said the strike had been “aimed at deterring future Iranian attack plans”.

However retaliation by Iran and its proxies against the US and its allies seems all but certain.

Oil prices rose 4 per cent amid fears of revenge attacks on refineries and other infrastructure.

The US said it would send an extra 3500 troops to the Middle East for 60 days as a precaution, joining around 5000 already in Iraq.

It was a dramatic escalation in tensions between Washington and Tehran, which have been engaged in a series of bloody tit-for-tat actions in recent weeks, and came just days after violent protests at the US embassy in Baghdad, which the US blamed on Soleimani.

Boris Johnson, who has spent the New Year holidaying in the Caribbean, appeared not to have been told of the attack in advance.

Foreign Secretary Dominic Raab appealed for a “de-escalation” of the tensions from all sides.

The Foreign and Commonwealth Office advised all UK citizens in Iran to “avoid any rallies, marches, or processions” in the coming days.

It said: “There is a risk that British nationals, and a significantly higher risk that British-Iranian dual nationals, could be arbitrarily detained or arrested in Iran.”

Labour leader Jeremy Corbyn last night demanded an urgent Privy Council meeting to discuss the fast moving situation.

He also wrote to the Prime Minister asking what the Government knew ahead of the air strike and if there was now an increased terror risk in the UK.

As people reacted on social media, “World World Three” trended alongside Franz Ferdinand, the archduke whose assassination in 1914 triggered World War One.

Soleimai, 62, was widely regarded as the second most powerful person in Iran after Ayatollah Khamenei, ahead even of the Iranian president Hassan Rouhani.

He had been head of the Quds force, an elite unit within the Iranian Revolutionary Guards, since 1998, running clandestine operations outside Iran and greatly expanding Iranian influence throughout the Middle East.

Under his leadership, Iran extended its military presence in Iraq, supported Hezbollah in Lebanon, and helped president Bashar al-Assad suppress the uprising in Syria.

He and officials from Iranian-backed militias had been leaving Baghdad airport in two cars when several missiles found them near a a cargo area.

Iran’s Revolutionary Guards said Iraqi militia leader Abu Mahdi al-Muhandis, whose group was blamed for the death of a US contractor in Iraq last week, was among at least seven killed.

Soleimani’s son-in-law and a member of the Lebanese Hezbollah were also among the dead, it added.

On Twitter, President Trump said: “General Qasem Soleimani has killed or badly wounded thousands of Americans over an extended period of time, and was plotting to kill many more...but got caught!

“He was directly and indirectly responsible for the death of millions of people, including the recent large number of PROTESTERS killed in Iran itself.

“While Iran will never be able to properly admit it, Soleimani was both hated and feared within the country.

“They are not nearly as saddened as the leaders will let the outside world believe. He should have been taken out many years ago!”

Mr Trump, who faces a Presidential election in November, previously accused his predecessor of stoking tension with Iran for political gain.

In 2011, he tweeted: “In order to get elected, Barack Obama will start a war with Iran.”

Former US vice president Joe Biden, who is hoping to be the Democratic challenger in the autumn, said: “President Trump just tossed a stick of dynamite into a tinderbox”.

In the US Congress, Senate Minority Leader Chuck Schumer said “no one should shed a tear” over Soleimani’s death, but criticised the lack of warning from President Trump.

UN Secretary-General Antonio Guterres said he was “deeply concerned”, and urged leaders to show “maximum restraint”.

His spokesman said the world “cannot afford” another Gulf war.

European leaders stopped short of criticising the US, and acknowledged Iranian provocation, but feared the development would be the final blow to the fragile Iranian nuclear deal.

Mr Trump pulled the US out of efforts to stop Iran’s nuclear weapons development, but EU countries had tried to stay engaged with Iran.

Iranian president Rouhani said Suleimani’s “martyrdom” would make the country more determined to resist America.

He said: “With no doubt, Iran and other freedom-seeking countries in the region will take his revenge.”

Mohammad Javad Zarif, the Iranian foreign minister, said on Twitter the killing was an “extremely dangerous & a foolish escalation”.

He warned: “The US bears responsibility for all consequences of its rogue adventurism.”

Tim Fletcher, former UK ambassador to Lebanon said Soleimani had been much more powerful than Osama bin Laden or Isis leader Abu Bakr al-Baghdadi at the time of their deaths by US forces, because he was still in the ascendant.

He said Iran had been “goading Washington, goading Donald Trump”, adding: “And of course, we don’t just have erratic leaders at the moment in Tehran, we have an erratic leader in Washington as well.”