The leader of Iran's elite Islamic Revolutionary Guard Corps (IRGC) has promised a "crushing response" to any American military attacks in the Gulf after United States President Donald Trump threatened to "shoot down and destroy" Iranian vessels.

Iran officials on Thursday also accused Trump of "bullying" and said he should focus on caring for US service members infected with the coronavirus instead of making threats. Thousands of US service members have contracted the virus, including hundreds on a stricken aircraft carrier, docked in Guam, and at least two have died from COVID-19.

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"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," General Hossein Salami told national TV. "Security of the Persian Gulf is part of Iran's strategic priorities."

The US president's tweet on Wednesday saying he had authorised the US Navy in the region to attack any Iranian fast boats that engaged American military assets came after an encounter last week between the two forces in the Gulf.

"US forces have no business 7,000 miles [12,500km] away from home, provoking our sailors off our OWN Persian Gulf shores," wrote Iran's Foreign Affairs Minister Javad Zarif on Twitter.

The US military is hit by over 5000 #covid19 infections. @realdonaldtrump should attend to their needs, not engage in threats cheered on by Saddam's terrorists.



Also, US forces have no business 7,000 miles away from home, provoking our sailors off our OWN Persian Gulf shores. pic.twitter.com/7CjzabkyVK — Javad Zarif (@JZarif) April 23, 2020

After Iran's announcement of the launch of a military reconnaissance satellite on Wednesday, Trump wrote on Twitter, without citing any specific incident: "I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea."

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

General Abolfazl Shekarchi, a spokesman for Iran's armed forces, called Trump "a bully" after issuing the threat.

At the Pentagon on Wednesday, the vice chairman of the Joint Chiefs of Staff, General John Hyten, welcomed Trump's tweet as a useful warning to Iran. He drew a parallel between last week's naval encounter in the Gulf and Wednesday's space launch, which he said was "just another example of Iranian malign behaviour".

"And it goes right along with the harassment from the fastboats ... You put those two things together and it's just more examples of Iranian malign behaviour and misbehaviour," Hyten said.

Iran considers the heavy US military presence in the Middle East a threat to its security.

'Right of self-defence'

Senior Pentagon officials gave no indication that Trump had directed a fundamental change in military policy on Iran.

"The president issued an important warning to the Iranians," David Norquist, the deputy secretary of defence, said when asked about the tweet. "What he was emphasising is all of our ships retain the right of self-defence."

But Representative Elaine Luria, a Democrat and Navy veteran, said Trump's tweeting could lead to war.

"The president's continued issuing of orders to our military via tweet is a threat to our national security and, if followed without clear guidance and rules of engagement, will unnecessarily escalate tensions with Iran and possibly lead to all-out-conflict," she said.

Asked whether the tweet means a repeat of last week's incident in the Gulf would require a lethal US response, Hyten said: "I would have to be the captain of the ship in order to make that determination."

Provocative launch

Iran's space launch has potentially bigger implications for conflict with Iran. US officials say it was intended to advance Iran's development of intercontinental-range ballistic missiles that could threaten the US.

Using a mobile launcher at a new site, the IRGC said it put the Noor, or "Light", satellite into a low orbit circling the Earth. Iranian national TV late on Wednesday showed footage of what it said was the satellite, and said it orbited the Earth within 90 minutes. National TV said the satellite's signals were being received.

Hyten said it was too soon to know whether the launch had successfully placed a satellite in orbit. He said US tracking technology showed the launch vehicle had travelled "a very long way, which means it has the ability once again to threaten their neighbors, their allies, and we want to make sure they can never threaten the United States".

Secretary of State Mike Pompeo said the United Nations needs to evaluate whether the space launch was consistent with Security Council resolutions.

"I don't think it remotely is and I think Iran needs to be held accountable for what it's done," Pompeo said.

In a letter on Wednesday to Trump, 50 former senior US officials and experts on Iran accused Tehran of using COVID-19 as a reason to pressure the US to ease sanctions, while continuing to spend money to bankroll "malign activities" in the region.

The letter called on Trump "to double down on the maximum pressure campaign to force the mullahs to spend their money on the Iranian people, not their nuclear ambitions, imperialism, and internal oppression".

Tensions between Iran and the US increased earlier this year after an American air strike killed Qassem Soleimani, head of Iran's elite Quds Force, in Iraq.

The assassination brought the two countries to the brink of war.

Iran retaliated on January 8 with a rocket attack on Iraq's Ain al-Assad base where US forces were stationed. No US troops were killed or faced immediate bodily injury, but more than 100 were later diagnosed with traumatic brain injuries.