Iranians pay homage to major general Qasem Soleimani on the streets of the city of Ahvaz after his death from an air-strike in Baghdad ordered by US President Donald Trump. Soleimani's killing ratcheted up tensions between the US and Iran and sparked fears of a new war in the Middle East. Photo: AFP

 Iran condemned Donald Trump on Sunday as a "terrorist in a suit" after the US president threatened to hit 52 Iranian sites hard if Tehran attacks Americans or US assets in retaliation for the killing of Iran's top military commander Qassem Soleimani.

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

Soleimani, Iran's preeminent military commander, was killed on Friday in a US drone strike on his convoy at Baghdad airport, an attack that took long-running hostilities between Washington and Tehran into uncharted territory and raised the specter of wider conflict in the Middle East.

"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,'" Iranian Information and Telecommunications Minister Mohammad Javad Azari-­Jahromi tweeted.

Soleimani was the architect of Tehran's overseas clandestine and military operations as head of the Revolutionary Guards' Quds Force. He was seen as Iran's second most powerful figure after Supreme Leader Ayatollah Ali Khamenei.

Khamenei promised on Friday that Iran would seek harsh revenge for his death.

Trump responded to that and other strong words from Tehran with a series of tweets on Saturday, saying Iran "is talking very boldly about targeting certain USA assets."

The US has "targeted 52 Iranian sites," 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," he said.

The 52 targets represented the 52 Americans held hostage in Iran after being seized at the US Embassy in 1979 during the country's Islamic Revolution, Trump added.

The two countries have no diplomatic relations and on Sunday, Iran summoned the Swiss envoy representing US interests in Tehran to protest at "Trump's hostile remarks," according to Iranian state television.

European Union foreign policy chief Josep Borrell urged Iran's foreign minister by phone on Sunday to work to de-escalate the situation and invited him to Brussels to discuss ways of preserving world powers' 2015 nuclear deal with Iran.