Iran has condemned Donald Trump 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 military commander Qassem Soleimani.
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.
Soleimani, Iran's pre-eminent 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 spectre 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'," 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. Supreme Leader Ayatollah Ali 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 United States 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 said.
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.
It was Trump's withdrawal of the United States from the deal in 2018 and reimposition of sanctions on Iran that touched off a new spiral of tensions after a brief thaw following the accord.
In Iraq, many people including opponents of Soleimani have expressed anger at Washington for killing him and Iraqi militia leader Abu Mahdi al-Muhandis on Iraqi soil and potentially dragging their country into another war.
Lawmakers planned a special parliamentary session on Sunday to push for a vote on a resolution requiring the government to ask Washington to withdraw US troops from the country.
"There is no need for the presence of American forces after defeating Daesh (Islamic State). We have our own armed forces which are capable of protecting the country," said Ammar al-Shibli, a member of parliament's legal committee.
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 attacks on US diplomats and military personnel. Democratic critics said the Republican president's action was reckless and risked more bloodshed in a dangerous region.