The two countries appear to be locked in a standoff over uranium enrichment nearly two months into the high-stakes negotiations.

Speaking Monday in a New York Post podcast, Trump vowed again to prevent Iran from acquiring a nuclear weapon.

When asked if he thought he could get Iran to agree to ends its nuclear program, Trump said: "I don't know."

"I don't know. I did think so, and I'm getting more and more -- less confident about it," the US leader told the interviewer.

He added: "They seem to be delaying and I think that's a shame. I'm less confident now than I would have been a couple of months ago. Something happened to them. But I'm much less confident about a deal being made."

Trump insisted that with or without an agreement, he would never let Iran acquire a nuclear weapon.

"Well, if they don't make a deal, they're not going to have a nuclear weapon," he said, according to the newspaper.

"If they do make a deal, they're not going to have a nuclear weapon, too, you know?"

Iran said Tuesday a sixth round of talks with the United States is planned for Sunday.

The longtime foes have held five rounds of negotiations since April, the highest level contact since Trump withdrew Washington from a 2015 nuclear accord during his first term.

Iran, in comments made days after Trump gave the interview, threatened on Wednesday to target US bases across the Middle East if conflict breaks out.

"All its bases are within our reach, we have access to them, and without hesitation we will target all of them in the host countries," Defence Minister Aziz Nasirzadeh said in response to US threats of military action if the talks fail.