Washington DC - President Donald Trump said US personnel were being moved from the potentially "dangerous" Middle East on Wednesday as nuclear talks with Iran faltered and fears grew of a regional conflict.

Trump also reiterated that he would not allow Iran to have a nuclear weapon, amid mounting speculation that Israel could strike Tehran's facilities.

Iran threatened Wednesday to target US military bases in the region if conflict breaks out.

A US official had earlier said that staff levels at the embassy in Iraq were being reduced over security concerns, while there were reports that personnel were also being moved from Kuwait and Bahrain.

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"Well, they are being moved out because it could be a dangerous place," Trump told reporters in Washington when asked about the reports of personnel being moved.

"We've given notice to move out, and we'll see what happens."

Trump then added: "They can't have a nuclear weapon, very simple. We're not going to allow that."

Tehran and Washington have held five rounds of talks since April to thrash out a new nuclear deal to replace the 2015 accord that Trump abandoned during his first term in 2018.

The two sides were due to meet again in coming days.

Trump had until recently expressed optimism about the talks, but said in an interview published Wednesday that he was "less confident" about reaching a nuclear deal.