Iran has hit back after Donald Trump said he had instructed the US Navy to fire on any Iranian ships that harass it at sea.
Tehran responded by saying it will destroy "any American terrorist force" if its security is threatened in the Gulf.
It comes as tensions escalate between the two countries again, with Iran's Republican Guard announcing the launch of the country's first military satellite.
I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea.— Donald J. Trump (@realDonaldTrump) April 22, 2020
Mr Trump said on Twitter: "I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea."
At a briefing later, he said that he was not changing the military's rules of engagement, adding: "We're covered - we're covered 100%".
He said: "We don't want their gunboats surrounding our boats and travelling around our boats and having a good time.
"We're not going to stand for it...they'll shoot them out of the water."
Responding on state TV, the head of Iran's elite Revolutionary Guards, Hossein Salami, said: "I have ordered our naval forces to destroy any American terrorist force in the Persian Gulf that threatens security of Iran's military or non-military ships.
"Security of the Persian Gulf is part of Iran's strategic priorities."
He added: "I am telling the Americans that we are absolutely determined and serious in defending our national security, our water borders, our shipping safety, and our security forces, and we will respond decisively to any sabotage.
"Americans have experienced our power in the past and must learn from it."
Last week, the US Navy said 11 Iranian gunboats had behaved in a "dangerous and harassing" way towards US vessels in the Persian Gulf. But Iran blamed the US.
Deputy Secretary of Defence David Norquist told reporters: "The president issued an important warning to the Iranians, what he was emphasising is all of our ships retain the right of self-defence."
General Abolfazl Shekarchi, a spokesman for Iran's armed forces, accused Mr Trump of "bullying" and said he should focus on his country's coronavirus problem.
The US has more than 840,000 confirmed cases of COVID-19, including 3,500 cases within the US military.
Meanwhile, there are fears that Iran's satellite launch will allow it to develop ballistic missiles capable of threatening the US, although Tehran denies this.
US Secretary of State Mike Pompeo said the launch could be a breach of a United Nations resolution which "called upon" Iran to stop working on missiles designed to carry nuclear weapons.
He said Iran needed to be "held accountable for what they have done", although some states argued that the language of the resolution did not make Iran's compliance obligatory.
It comes after a number of recent events have tested relations between the two countries.
On 3 January, the US killed Qassem Soleimani, the head of Iran's elite Quds Force, in a drone strike in Iraq.
Iran retaliated five days later with a rocket attack on an Iraqi base housing US troops.
In 2018, Mr Trump pulled out of the nuclear deal between Iran and other world powers, reimposing sanctions on Iran.
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