Donald Trump is vowing instant retaliation if Iran responds in kind to the US assassination of Qassem Soleimani, promising to pound 52 Iranian sites – one for each of the Americans held hostage in Tehran after the 1978 embassy takeover.

The Iraqi militia leader and mastermind of Iran’s regional security strategy was blown to pieces by a drone strike on Friday as his entourage motored from Baghdad airport.

Hundreds of thousands of Iranians gathered for a funeral procession in the town of Ahvaz to Soleimani, holding portraits of the popular commander.

A sea of mourners pays homage to top Iranian general Qasem Soleimani. Photo: Getty

Mr Trump warned that the US was ready to escalate if Tehran strikes back, saying the US had already “targeted 52 Iranian sites, some at a very high level and important to Iran & the Iranian culture.”

He did not identify the targets but added that they would be hit “very fast and very hard”.

….targeted 52 Iranian sites (representing the 52 American hostages taken by Iran many years ago), some at a very high level & important to Iran & the Iranian culture, and those targets, and Iran itself, WILL BE HIT VERY FAST AND VERY HARD. The USA wants no more threats!

— Donald J. Trump (@realDonaldTrump) January 4, 2020

Iran has condemned Mr Trump as a “terrorist in a suit”.

“Like ISIS, Like Hitler, Like Genghis! They all hate cultures. Trump is a terrorist in a suit. He will learn history very soon that NOBODY can defeat ‘the Great Iranian Nation & Culture’,” Information and Telecommunications Minister Mohammad Javad Azari-Jahromi tweeted.

As the two countries assailed each other in a war of words, the European Union, Britain and Oman urged the parties to seek to de-escalate the crisis.

Mr Trump says he ordered the strike, a high-risk decision that was made without consulting Congress or US allies, to prevent a conflict.

US officials say Soleimani was plotting a series of attacks that would have endangered American troops and officials – a claim for which he provided no evidence.

Soleimani has been blamed for attacks on US troops and American allies going back decades.

Though it’s unclear how or when Iran may respond, any retaliation is likely to come after three days of mourning declared in both Iran and Iraq. All eyes were on Iraq, where America and Iran have competed for influence since the 2003 US-led invasion.

After the airstrike, the US-led coalition has scaled back operations and boosted “security and defensive measures” at bases hosting coalition forces in Iraq, a coalition official said on the condition of anonymity according to regulations.

Meanwhile, the US has dispatched another 3,000 troops to neighbouring Kuwait, the latest in a series of deployments in recent months as tensions with Iran have worsened.

As the funeral processions were still taking place, a series of rocket attacks on bases that house US forces were launched.

A Katyusha rocket that fell inside a square less than a kilometre from the US Embassy, according an Iraqi security official who spoke on condition of anonymity. The US-led coalition confirmed the attack, with a spokesman confirming no troops were hurt.

Rockets also landed in the vicinity of Balad air base, about 65 kilometres north of Baghdad, the coalition said, also with no resulting injuries. Earlier an Iraqi official said three rockets fell in a farm area outside the base.

In Baghdad, thousands of mourners, mostly men in black military fatigues, carried Iraqi flags and the flags of Iran-backed militias that are fiercely loyal to Soleimani at Saturday’s ceremony. They were also grieving for Abu Mahdi al-Muhandis, a senior Iraqi militia commander who was killed in the same strike.

The slain Iraqi militants will be buried in Najaf, while Soleimani’s remains will be taken to Iran. More funeral services will be held for Soleimani in Iran before his body is laid to rest in his hometown of Kerman.

The US has ordered all citizens to leave Iraq and temporarily closed its embassy in Baghdad, where Iran-backed militiamen and their supporters staged two days of violent protests in which they breached the compound.

Britain and France have warned their citizens to avoid or strictly limit travel in Iraq.

-with AAP