US President Donald Trump says the Middle East is facing the "chance of massive conflict" after Iran revealed it had received a warning that Israel was planning to attack it.
Regional tensions are rising after a new UN watchdog finding that Iran was not complying with commitments designed to prevent it from building a nuclear weapon.
Iran responded angrily and announced it would activate a new nuclear enrichment facility, which would be the country's third.
"The Islamic Republic of Iran has no choice but to respond to this political resolution," the Iranian Foreign Ministry and Atomic Energy Organization said in a joint statement.
Separately, an Iranian official said a "friendly country" had warned Tehran that Israel was planning to strike its nuclear facilities.
The US on Wednesday, local time, said it was ordering some American personnel to move out of the region, including all non-essential staff from its embassy in Baghdad.
The State Department also gave non-essential staff the option to leave Bahrain and Kuwait. There are fears US facilities could be the target of Iranian retaliation to any attack.
On Thursday, Mr Trump said: "Look, there's a chance of massive conflict.
"We have a lot of American people in this area. And I said, we gotta tell them to get out because something could happen. Soon.
"And I don't want to be the one that didn't give any warning and missiles are flying into their buildings. It's possible."
Asked how imminent an Israeli strike on Iran was, he said: "I don't want to say imminent, but it looks like something that could very well happen."
The US has been attempting to negotiate a deal with Iran that could see Tehran rein in its nuclear program in exchange for the lifting of sanctions, which have crippled its economy.
Oman has confirmed it will mediate further talks between the US and Israel this weekend. The success or failure of the talks could influence Israel's next move.
Mr Trump said he believed an agreement with Iran was "fairly close".
"As long as there is an agreement, I don't want them [Israel] going in [to Iran] because I think it would blow it," Mr Trump said. "Might help it, actually, but it also could blow it."
Talks to resume as world considers nuclear findings
In 2018, the first Trump administration pulled the US out of an existing deal with Iran, and Iran has rapidly advanced its uranium enrichment program in the years since.
But the UN watchdog's declaration that Iran is not complying with its non-proliferation obligations is the first finding of its kind in almost 20 years.
The watchdog, the International Atomic Energy Agency (IAEA), warned that Iran had amassed more near-weapons-grade uranium and was now "the only non-nuclear-weapon state to produce such material".
It could mean Iran is referred to the UN Security Council and penalised with further sanctions.
Mr Trump has previously warned that either Israel or the US could strike Iran's nuclear facilities if Iran does not agree to a nuclear deal.
On Thursday evening, he wrote on social media that his "entire administration has been directed to negotiate with Iran".
US Middle East envoy Steve Witkoff will attend the next round of talks between Iran and the US, which are scheduled to start in Oman on Sunday.
