Donald Trump said he was considering "blowing up the entire nation" of Iran amid claims the US was preparing to resume military action against the regime.
The US president also suggested he could "let them rot economically", as Tehran is squeezed by a naval blockade, or strike a peace deal.
Iran on Sunday said it was preparing for possible air strikes and threatened to retaliate. US embassies and the US state department issued security alerts this weekend amid fighting between Saudi Arabia and Yemen's Houthis.
Mr Trump, who left the presidential retreat at Camp David earlier than expected on Saturday to return to the White House, told Fox News on Sunday he was in a "deciding mode" and that "very big things are going to be happening in the not-so-distant future".
His options were "wiping Iran out, letting them rot economically or making a deal", he said, adding: "My question is if and when do I blow the entire nation up?"
A billboard in Tehran bearing the slogan 'Great Victory ' was installed in August after the US announced a campaign of 'economic exclusion' - Fatemeh Bahrami/Anadolu via Getty Images
It marks a return to the president's more aggressive rhetoric as a peace deal or decisive victory against Tehran continues to elude him.
He has previously threatened to "blow… up the entire country", bomb Iran "back to the Stone Ages", and declared in May: "A whole civilisation will die tonight, never to be brought back again."
JD Vance, the US vice-president, and Marco Rubio, the US secretary of state, have reportedly warned the president that the war could last for the duration of his term, which is set to end in January 2029. However, such threats have been sporadic since the US and Iran agreed a ceasefire on April 8.
Iran's military command said on Sunday it had received intelligence that the US was planning to act in concert with other Middle Eastern nations to resume military action, which has been largely sporadic since early April.
The regime threatened sustained strikes against US bases, interests and allies in the region in response.
Mr Trump said he was "probably" open to meeting Masoud Pezeshkian, the Iranian president, when they both attend the UN General Assembly in New York, which begins on Tuesday.
Previous attempts at negotiation have been frustrated by control over the Strait of Hormuz, over which Iran claims sovereignty, and tensions between Iran's moderate civilian leaders and military hardliners.
Mr Trump criticised Iranian officials, whom he accused of "hiding like rats", and said no direct communications were taking place between the US and Iran.
The president also appeared to signal he would not intervene in the conflict between the Houthis and Saudi Arabia, saying the Iranian proxy group had "agreed not to fight the United States".
Despite alerts issued by the US state department and embassies, he claimed it was "not different from any other weekend".
The Houthis seized the Red Sea coastline and Bab al-Mandeb Strait in a lightning military offensive this month, meaning they could potentially choke off shipping through the waterway and inflict further damage on the world economy.
On Saturday, the rebel group launched a barrage of drones and missiles at Riyadh and oil facilities, marking a significant escalation in the conflict.
American embassies across the Middle East issued security alerts for US citizens early on Sunday morning, warning of "potential for unforeseen escalation" and urging "heightened vigilance".
"Be aware of potential flight cancellations, airspace closures, and travel disruptions," the notice added.
The US state department on Saturday warned Americans against travelling to the Middle East, saying the conflict between the Houthis and Saudis "has the potential to escalate rapidly".
John Ratcliffe, the CIA director, met Abdel Fattah al-Sisi, the Egyptian president, during a rare visit to Cairo on Sunday.
According to Mr Sisi's office, they discussed the war in Iran, attacks in the Gulf and the threats to maritime security. The Egyptian president is said to have stressed "the paramount importance of settling regional crises through political and diplomatic tracks".
