Donald Trump said on Thursday night that it was the "calm before the storm" amid reports that he will announce next week that he will “decertify” the Iran nuclear deal.
The US president earlier said that Iran had "not lived up to the spirit" of the nuclear deal.
Mr Trump summoned reporters to the State Dining Room on Thursday evening, where he was hosting a dinner with military leaders.
"You guys know what this represents?" Mr Trump asked. "Maybe it's the calm before the storm. Could be the calm, the calm before the storm."
"What storm Mr. President?" one reporter shouted. ISIS? North Korea? Iran?
"You'll find out," the president said.
A move to end the Iran nuclear deal could lead to renewed US sanctions against Tehran, unnamed senior administration officials said.
Mr Trump is expected to give a major foreign policy speech on October 12, in which he will blame Iran for fuelling terrorism and causing instability throughout the Middle East. The speech is said to mark a shift to a more confrontational policy towards Tehran.
The US president said on Thursday evening that Iran had not lived up to the spirit of the nuclear deal. "We must not allow Iran ... to obtain nuclear weapons," Mr Trump said during a meeting with military leaders at the White House.
"The Iranian regime supports terrorism and exports violence, bloodshed and chaos across the Middle East. That is why we must put an end to Iran's continued aggression and nuclear ambitions."
Asked about whether he would "decertify" the deal, he said: "You'll be hearing about Iran very shortly."
Sarah Huckabee Sanders, the White House press secretary, said: "The president is going to make an announcement about the decision that he's made on a comprehensive strategy that his team supports, and we'll do that in the coming days."
If Mr Trump states that Iran has not been complying with the terms of the nuclear deal, designed to prevent the country obtaining a nuclear weapon, Congress would then have 60 days to decide whether to reimpose sanctions that were suspended under the agreement.
The US president has been a frequent critic of the Iran nuclear pact, a signature foreign policy achievement of his Democratic predecessor, Barack Obama, and signed in 2015 by the United States, Britain, France, Germany, Russia, China, the European Union and Iran.
Under the agreement, which was signed in Vienna in 2015, Iran agreed to sacrifice two-thirds of its ability to enrich uranium in return for an ending of economic sanctions.
Mr Trump recently called it “one of the worst deals I’ve ever seen”, stating that Iran had “violated so many different elements, but they’ve also violated the spirit of that deal”.
The move to end the deal has been widely opposed by European leaders. The British, French, German and European Union ambassadors to the US participated in a meeting on Capitol Hill on Wednesday that spelled out the consequences of the collapse of the deal.
Last month, Boris Johnson, the Foreign Secretary, urged the world to have faith in the potential of the deal to create a more open Iran.
While Mr Johnson warned Iran "to stop their adventurist and expansionist plans, causing trouble in Yemen, Syria or anywhere else", he argued the deal could liberate the Islamic republic.
“On the other side, we in the UK feel that Iran – a country of 80 million people, many of them young and potentially liberal – could be won over. I think it is important they see there are benefits from the JCPOA (an acronym for the nuclear agreement), so we in the UK want that alive,” Mr Johnson said, speaking alongside Rex Tillerson, his US counterpart.
Supporters of the deal say its collapse could trigger a regional arms race and increase tensions in the Middle East.
Opponents say it went too far in easing sanctions without requiring that Iran end its nuclear programme permanently.
The White House did not confirm the reported plans for Mr Trump's speech. Mr Trump has to report to Congress on Oct 15 whether he believes Iran is complying with the nuclear agreement.
Michael A. Anton, spokesman for the White House national security council, told the Washington Post: “The administration looks forward to sharing details of our Iran strategy at the appropriate time.”
Hassan Rouhani, the Iranian president, said last month he would not renegotiate the terms of the deal. He warned that cancelling the deal "would carry a high cost for the United States of America".