Iranians march on January 5, 2020 in the streets of the northwestern city of Ahvaz to pay homage to top general Qasem Soleimani, after he was killed in a US strike in Baghdad.

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

"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.

Soleimani, Iran's pre-eminent military commander, was killed on Friday in a U.S. drone strike on his convoy at Baghdad airport, an attack that has taken long-running hostilities between Washington and Tehran into uncharted territory and raised the spectre of wider conflict in the Middle East.

Soleimani was the architect of Tehran's overseas clandestine and military operations as head of the Revolutionary Guards' Quds Force.

Giving no indication of seeking to defuse tensions after the strike he ordered, Trump issued a stern threat to the Islamic Republic.

In a series of tweets on Saturday he said Iran "is talking very boldly about targeting certain USA assets" to avenge Soleimani's death. Trump said the United States has "targeted 52 Iranian sites" and that some were "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 U.S. Embassy in 1979 during the country's Islamic Revolution.

Iran's army chief, Major General Abdolrahim Mousavi, was quoted by state television on Sunday as saying the United States lacked the courage for military confrontation with Iran.

"In a potential conflict in the future, which I don't think they (Americans) have the courage to carry out, there it will become clear where the numbers five and two will belong," he said.

Trump said on Friday Soleimani had been plotting "imminent and sinister" attacks on U.S. diplomats and military personnel. Democratic critics said the Republican president's action was reckless and risked more bloodshed in a dangerous region.

Thousands of mourners turned out to pay respects to the slain commander on Sunday after Soleimani's body was returned to Iran, the official IRIB newsagency reported.