The sixth round of US-Iran nuclear talks will be held on Sunday in Muscat, the Omani foreign minister said on Thursday, after US President Donald Trump reiterated that Tehran would not be allowed to have a nuclear weapon.

Trump said on Wednesday US personnel were being moved out of the Middle East because "it could be a dangerous place".

The United States has been reported as preparing an evacuation of its Iraqi embassy and allowing military dependents to leave locations around the Middle East due to heightened security risks in the region, according to US and Iraqi sources.

The four US and two Iraqi sources did not say what security risks had prompted the decision.

Reports of the potential evacuation pushed up oil prices by more than 4 percent before prices eased on Thursday.

Foreign energy companies were continuing their operations as usual, a senior Iraqi official overseeing operations in southern oilfields said on Thursday.

A US official said the State Department had authorized 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 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 US 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 said.

"We've given notice to move out."

Asked whether anything could 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 in an interview released earlier on Wednesday said he was growing less confident that Tehran would agree to stop enriching uranium, a key American demand.

While the evacuation of non-essential personnel raised concerns about a possible regional escalation, a senior Iranian security official told Iran's Press TV on Thursday that US military dependents leaving did not constitute a threat.

On Wednesday, Iran's defence minister warned Washington that Tehran would hit US regional bases if drawn into a war in the case of nuclear talks failing.

The United States has a military presence across the major oil-producing region, with bases in Iraq, Kuwait, Qatar, Bahrain and the United Arab Emirates. (Reuters)