US President Donald Trump tweeted on Wednesday morning that he had told the United States Navy to "shoot down and destroy" Iranian gunboats that "harass" US ships.

"I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea," Trump tweeted.

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Trump did not cite a specific event in his tweet or provide details.

He said later on Wednesday that US Navy ships would shoot Iranian gunboats that get too close "out of the water".

"We're not going to stand for it. If they do that that's putting our ships in danger and our great crews and sailors in danger - I'm not going to let that happen. And we will - they'll shoot them out of the water," he said during a White House briefing. "That's a threat. When they get that close to our boat, and they have guns, they have very substantial weapons on those boats, but we'll shoot them out of the water."

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

Senior Pentagon officials said Trump's comments were meant as a warning to Tehran but suggested that the US military would continue to abide by their existing right to self defence instead of changing their rules.

"The president issued an important warning to the Iranians, what he was emphasising is all of our ships retain the right of self-defence," Deputy Secretary of Defense David Norquist told reporters at the Pentagon.

Following Trump's tweet, an Iranian armed forces spokesman said the US should focus on saving its military from the coronavirus.

“Today, instead of bullying others, the Americans should put all their efforts toward saving those members of their forces who are infected with coronavirus,” Abolfazl Shekarchi said, according to Iran's semi-official ISNA news agency.

Trump's comments on Wednesday came about a week after 11 vessels from the Iranian Revolutionary Guards Corps Navy came close to US ships.

On Sunday, the Revolutionary Guard acknowledged it had a tense encounter last week with US warships in the Gulf, but alleged without offering evidence that US forces sparked the incident.

Iranian and US vessels routinely have tense encounters in the Gulf and its narrow mouth, the Strait of Hormuz, through which 20 percent of the world's oil passes.

Satellite launch

Earlier on Wednesday, the Revolutionary Guard said it had put the country's first military satellite into orbit, dramatically unveiling what experts described as a secret space programme with a surprise launch that came amid wider tensions with the US.

The satellite "orbited the Earth at 425km [264 miles]", said the elite forces' official website. "This action will be a great success and a new development in the field of space for Islamic Iran."

The launch immediately raised concerns among experts about whether the technology used could help Iran develop intercontinental ballistic missiles.

US Secretary of State Mike Pompeo said that "Iran needs to be held accountable for what they've done".

"They have now had a military organisation that the United States has designated a terrorist attempt to launch a satellite," Pompeo said at a news conference.

Tensions between the US and Iran have escalated since Trump pulled the US out of the landmark Iran nuclear deal in 2018 and reimposed crippling sanctions.

Earlier this year, the US killed Qassem Soleimani, the head of Iran's elite Quds Force, in a drone strike in Iraq.

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 injury.