Washington DC - President Donald Trump said Friday he was frustrated by Iran's position in talks but added he had not yet decided whether to carry out a threatened attack.

A day after the US and Iran held talks in Geneva, Trump said that the cleric-run state was "not willing to give us what we have to have."

"We haven't made a final decision," he told reporters when asked about the use of force.

"We're not exactly happy with the way they negotiated. They cannot have nuclear weapons, and we're not thrilled with the way they're negotiating," Trump said.

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"We want no nuclear weapons by Iran, and they're not saying those golden words."

Iran has said repeatedly that it is not pursuing nuclear weapons, and US intelligence has found no evidence that it has made a decision to do so.

But the US and Israel, which carried out a major bombing campaign in Iran in June, are skeptical of the statements by Iran, which backs Palestinian militants Hamas.

Asked if an attack would trigger all-out war in the Middle East, Trump said, "I guess you could always say there's always a risk."

He continued, saying, "You know when there's war, there's a risk in anything, both good and bad."

Trump was circumspect on whether a US attack would bring down the Islamic republic, a sworn enemy of the US and Israel.

Authorities last month killed thousands of people as they crushed mass protests in the biggest threat to the ruling clerics since the 1979 Islamic revolution toppled the pro-Western shah.

"Nobody knows. There might be, and there might not be," Trump said of regime change.

Trump in his first term ripped up a nuclear deal negotiated by his predecessor Barack Obama in which Iran agreed to strict limits on uranium enrichment.