US President Donald Trump has said US personnel were being moved out of the Middle East because "it could be a dangerous place," adding that the United States would not allow Iran to have a nuclear weapon.

On Thursday, The United States Embassy in Israel issued a security alert restricting the movement of its employees and their family members in the country, amid escalating tensions in the region.

The alert was issued amid reports in the US media during the last few days that Israel was planning a military strike against Iran to foil its nuclear programme.

US and Iraqi sources did not say what security risks had prompted the decision and reports of the potential evacuation pushed up oil prices by more than 4 per cent.

A US official said the State Department had authorised voluntary departures from Bahrain and Kuwait.

The State Department updated its worldwide travel advisory on Wednesday evening to reflect the latest US posture. “On June 11, the Department of State ordered the departure of non-emergency US government personnel due to heightened regional tensions,” the advisory said.

The decision by the US to evacuate some personnel comes at a volatile moment in the region. Trump's efforts to reach a nuclear deal with Iran appear to be deadlocked and U.S. intelligence indicates that Israel has been making preparations for a strike against Iran's nuclear facilities.

"They are being moved out because it could be a dangerous place, and we'll see what happens," Trump told reporters. "We've given notice to move out."

Asked whether anything can be done to lower the temperature in the region, Trump said: "They can't have a nuclear weapon. Very simple, they can't have a nuclear weapon."

Trump has repeatedly threatened to strike Iran if stuttering talks over its nuclear programme fail and earlier on Wednesday said he was growing less confident Tehran would agree to stop enriching uranium, a key American demand.