The South Carolina Republican, one of the president's most dutiful supporters, fired off his bellicose tweet after getting a briefing from National Security Adviser John Bolton, an Iran hawk who has long argued for regime change in the country.
Romney, a member of the Senate foreign relations committee, said the threat to U.S. interests was "real" and added: "We're going to make sure they understand that if they take action against our people, against our allies and against our friends, there will be effect and it will be far more severe than the initial action taken by Iran".
Trump's tweet immediately followed news that a rocket had landed in Baghdad's Green Zone, not too far from the US embassy.
The crisis is rooted in President Donald Trump's decision to withdraw from the nuclear accord previous year and impose sweeping US sanctions on Iran.
Both Iranian and US officials have said repeatedly during the past ten days that although they do not want a war with the other side, they would retaliate if their interests are attacked. "Never threaten the United States again", warned the U.S. president.
Several worldwide vessels were sabotaged in recent weeks, and the White House has pointed the finer at Iran, which backs several militias involved in military conflicts in the region.
Saudi Arabia's minister of state for foreign affairs, Adel al-Jubeir, said Sunday his country does not want to go to war with Iran but would defend itself.
Trump in recent days downplayed any potential for conflict, though he warned Sunday that the USA could destroy the country if provoked.
"I would say to the Iranians, do not underestimate the resolve on the USA side in the situation", British foreign minister Jeremy Hunt told reporters on Monday in Geneva.
Iran has increased the enrichment of low-grade uranium by fourfold as it says stockpile limits under the 2015 nuclear deal no longer apply, the Iranian Tasnim news agency has said, a week after Iran officially stopped some commitments under an worldwide accord.
Hunt said "They don't want a war with Iran", However, he warned, "But if American interests are attacked, they will retaliate". This account has been met with widespread scepticism outside the United States.
Trump's tweet was a "genocidal taunt", according to Iran's top diplomat, Zarif.
Saudi Arabia has accused Tehran of ordering last week's drone strikes on two oil pumping stations in the kingdom, an attack claimed by Iran-backed Huthi rebels in Yemen.
The Trump administration last week ordered non-essential diplomatic staff out of Iraq, citing the danger posed by Iranian-backed Iraqi armed groups.
On the Iranian side, the head of the country's elite Revolutionary Guard, Gen. Hossein Salami, was quoted Sunday as saying Iran is not looking for war.
King Salman invited Gulf leaders and Arab League member states to two emergency summits in Mecca on May 30 to discuss recent "aggressions and their consequences", the kingdom's official SPA news agency reported late Saturday.
The kingdom's regional allies welcomed the Saudi invitation.
Oil supplies are sufficient and stockpiles still rising despite massive output drops from Iran and Venezuela, said Saudi Arabia and the United Arab Emirates said at the meeting in Jeddah. That would potentially bring it closer to being able to develop a nuclear weapon, something Iran insists it has never sought. The U.S. responds by imposing sanctions on Iran's metals industry.
Iranian officials have repeatedly slammed the unilateral United States sanctions as "economic terrorism", saying that they have impeded the flow of essential goods.