US President Donald Trump is planning to abandon the Iran nuclear deal shortly, according to US media reports.
If he fails to certify the accord, Congress will decide whether to re-impose economic sanctions on Iran. Mr Trump has until 15 October to decide.
Opposition to the deal was a major part of his campaign last year.
Posing for photographers with military leaders on Thursday, he said this was “the calm before the storm” but refused to give further details.
There was speculation his comments might refer to heightened tensions with North Korea, but the New York Times says “people who have been briefed on the matter” believe he means Iran.
“Maybe it’s the calm before the storm,” he said.
When reporters pressed him on what storm he was referring to, he would only say: “You’ll find out.”
He had earlier told his top defence officials he expected them to provide “a broad range of military options… at a much faster pace” in future.
What happens next?
US media say the president will announce next Thursday that he will not be certifying the deal on the grounds it does not serve US security interests.
But some of his top advisers, such as Defence Secretary James Mattis, appear to back the deal.
The presidency must certify the deal every 90 days; Mr Trump has already done so twice.
If he rejected it this time, Congress would have 60 days to decide whether to re-impose sanctions on Iran. US media reports suggest they are likely to leave the deal in place.
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Speaking in the White House’s Cabinet Room, President Trump said: “The Iranian regime supports terrorism and exports violence and chaos across the Middle East.
“That is why we must put an end to Iran’s continued aggression and nuclear ambitions. You will be hearing about Iran very shortly.”
In reaction, Iranian nuclear chief Ali Akbar Salehi said the deal was not renegotiable, reported Iran’s Press TV.
But he suggested it might be salvageable if the other partners – France, Germany, China, Russia and the UK – remained on board. If not, he said, the deal will “definitely fall apart”.
Russian Foreign Minister Sergei Lavrov said he hoped “the final decision reached by the US president will be balanced and will be based on the present-day realities”, reported Interfax news agency.
Seeking to have his cake and eat it?
Analysis by the BBC’s Jonathan Marcus
President Trump looks set to light a fuse which could explode the whole nuclear deal with Iran.
A determination by the president that the agreement is no longer in America’s national interest would hand the whole issue to Congress.
If US legislators decide to re-impose some or all of the economic sanctions against Iran waived under the agreement, then the US will no longer be in compliance with it. That could lead to diplomatic tensions, rows over new sanctions and possibly encourage Iran to re-start its banned nuclear activities.
There is no doubt – even among Mr Trump’s most senior officials – that Iran is complying with the agreement in full.
By handing the problem over to Congress Mr Trump may be seeking to have his cake and eat it – demonstrating his own abiding hostility to the deal while perhaps not destroying it altogether.
What is the Iran nuclear deal?
The 2015 Joint Comprehensive Plan of Action was designed to prevent Iran developing a nuclear weapon, with the president’s administration having to certify to Congress that Iran is upholding its part of the deal every 90 days.
It lifted some sanctions that stopped Iran from trading on international markets and selling oil.
The lifting of sanctions is dependent on Iran restricting its nuclear programme. It must restrict its uranium stockpile, build no more heavy-water reactors for 15 years, and allow inspectors in to the country.
Mr Trump has repeatedly said Iran has broken the “spirit” of the deal.
In a speech to the UN General Assembly, Mr Trump called the deal, which was brokered while his predecessor Barack Obama was in power, “an embarrassment to the United States”.