Washington DC - President Donald Trump said Wednesday that Iran's supreme leader Ayatollah Ali Khamenei should be "very worried," as Washington builds up its military forces in the region.

"I would say he should be very worried, yeah, he should be," Trump said in an interview with NBC News.

"As you know, they are negotiating with us."

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Trump's comments came as a report by the Axios news outlet said that US-Iran talks planned for Friday were "collapsing" after US officials declined to move the location of the talks or shift the format.

The White House did not immediately comment on the Axios report when asked by AFP.

Trump has sent a US aircraft carrier to the region and has not ruled out new military action to follow the US attacks on Iranian nuclear sites during Israel's June war against the Islamic Republic.

Trump also said that Iran had eyed a new nuclear site after US strikes.

"They were thinking about starting a new site in a different part of the country," Trump told NBC.

"We found out about it, I said, you do that, we're going to do very bad things to you."