The White House last night issued a warning to Iran that "there's a new president in town" and it would not "sit by" and allow the country to pursue its military ambitions.
Diplomatic tensions were ratcheted up after Iran's Supreme Leader Ayatollah Ali Khamenei made his first public speech since President Donald Trump's inauguration.
He dismissed the US decision to put Iran "on notice" over a recent ballistic missile test and called on Iranians to take part in demonstrations on Friday, when the anniversary of the 1979 Islamic revolution will be marked.
Ayatollah Khamenei said:"We are thankful to (Trump) for making our life easy as he showed the real face of America. "He says 'You should be afraid of me'. No! The Iranian people will respond to his words on February 10 and will show their stance against such threats."
Mr Trump responded to the January 29 missile test by saying Iran was "playing with fire" and imposed fresh sanctions on individuals, some of them linked to Iran's elite Revolutionary Guard.
The missile test was not a direct breach of Iran's 2015 nuclear pact with six world powers, but the White House said it "violated the spirit of that"..
Last night Mr Trump's spokesman Sean Spicer re-iterated his firm stance.
"I think the Ayatollah is going to realise there's a new president in office," Mr Spicer said.
"This president's not going to sit by and let Iran flout its violations, or its apparent violations, to the joint agreement, but he will continue to take action as he sees fit.
"The president has also made clear time and time again that he's not going to project what those actions will be, and he's not going to take anything off the table.
But I think Iran is kidding itself if they don't realise there's a new president in town."
Mr Trump was also facing a legal showdown over his travel and refugee ban and vowed to take it all the way to the US Supreme Court if necessary.
He said: "We’re going to take it through the system. It’s very important for the country. Regardless of me or whoever succeeds at a later date.
"If you remember Isis said 'We are going to infiltrate the United States and other countries through the migration'. And then we’re not allowed to be tough on the people coming in? Explain that one."
Mr Trump added: "It's common sense. Some things are law, I'm all in favour of that. Some things are common sense, and this is common sense. And a lot of people agree with us, believe me."
Mr Trump signed his executive order temporarily banning people from seven predominantly Muslim countries entering the US, and suspending the US refugee programme, on Jan 27.
Seattle US District Judge James Robart later temporarily blocked the order.
The US government was asking an appeal court to restore it.
The hearing was due to unfold before a randomly selected panel of judges from the San Francisco-based 9th US Circuit Court of Appeals.
Whatever the appeals court decides either side could ask the Supreme Court to intervene.
Accusing the media of under-reporting terror attacks Mr Spicer said: "We need to remind people that the Earth is a very dangerous place these days. The president is trying to keep us safe."