President Donald Trump has warned that the US will hit 52 Iranian targets "very fast and very hard" if the country retaliates for the killing of one of its top commanders.
General Qassem Soleimani, who was in charge of Iran's military operations abroad, was killed in a US air strike at Baghdad airport on Friday.
In a series of tweets, Mr Trump said his administration had targeted 52 Iranian sites, "some at a very high level & important to Iran & the Iranian culture".
He linked the number of sites to the number of hostages, also 52, currently held by the Islamic Republic.
It comes as hundreds of thousands of people gathered on the streets of Iran to mourn the death of Gen Soleimani.
Iran vowed to deliver "severe revenge" for the attack but Mr Trump warned: "The USA wants no more threats!"
He went on to say the US has recently spent "two trillion dollars on military equipment" and that the US military is "the biggest and by far the BEST in the world!".
"If Iran attacks an American Base, or any American, we will be sending some of that brand new beautiful equipment their way...and without hesitation!," Mr Trump said.
Iran's foreign minister Mohammad Javid Zarif responded to Mr Trump's tweets, saying the killing of Gen Soleimani was a breach of international law.
He also said that targeting any cultural Iranian sites is "a war crime".
Both the UK's Defence Secretary and Foreign Secretary have called on "all parties" to "de-escalate".
Meanwhile, Royal Navy warships are to restart escorting British ships through the Strait of Hormuz, amid the heightened tensions.
Boris Johnson, who has been celebrating the New Year in the Caribbean, is yet to comment on the attack.
Foreign Secretary Dominic Raab said the Prime Minister is "in charge", despite being away from Number 10.
Labour's shadow foreign secretary Emily Thornberry accused the PM of "sunning himself drinking vodka martinis somewhere else and not paying attention".
Cover Image: US President Donald Trump has warned Iran after the country threatened to retaliate for the killing of General Qassem Soleimani (Picture: PA).