- Iran had agreed to limit nuke program in 2015 to ease economic sanctions
- US's move to decertify deal means its keeping agreement alive for now
- Trump had earlier termed pact as 'worst deal ever, an embarrassment'
Donald Trump will make the announcement in a speech at 12:45 pm EDT (1645 GMT) that will detail a more confrontational approach to Iran over its nuclear and ballistic missile programs and its financial and military support for extremist groups in the Middle East, US Secretary of State Rex Tillerson told reporters.
Trump will also give the US Treasury Department broad authority to impose economic sanctions against people and entities in the Iranian military, the Iran Revolutionary Guard, in response to what Washington calls its efforts to destabilize and undermine its opponents in the Middle East.
Trump's decision to decertify the nuclear deal will not withdraw the United States from the agreement, which was negotiated by the United States and other world powers during the administration of former President Barack Obama.
Trump will attempt to persuade the US Congress to approve some separate measures to toughen US policy toward Iran.
Tillerson acknowledged the strategy may not work.
"What we are laying out here is this is the pathway we think provides us the best platform from which to attempt to fix this deal," he said. "We may be unsuccessful. We may not be able to fix it. And if we're not then we may end up out of the deal."
Trump's move will put Washington at odds with other signatories of the accord, which include Britain, France, Germany, Russia, China, the European Union and Iran.
It will add to actions the Trump administration has taken in pursuit of an "America First" approach to international agreements, such as his decision to withdraw from the Paris climate accord and abandoning the Trans-Pacific Partnership trade talks.
The Republican president has been under strong pressure in recent weeks from European leaders and US lawmakers to swallow his concerns and certify the nuclear deal because international inspectors say Iran is in compliance with it.
European officials have categorically ruled out renegotiating the deal, but have said they share Trump's concerns over Iran's destabilizing influence in the Middle East.
'WORST DEAL EVER'
The threat of new action from Washington has prompted a public display of unity from the rival factions among Iran's rulers.
Trump, who took office in January, had reluctantly certified the deal twice before and has repeatedly blasted the agreement as "the worst deal ever" and an embarrassment.
His decision will set in motion a 60-day countdown for the US Congress to decide whether to reimpose US economic sanctions on Tehran that were suspended under the agreement.
If Congress reimposed the sanctions, the United States would then be in violation of the terms of the nuclear deal and it would likely fall apart. If Congress does nothing, the deal remains in place.
Tillerson said that rather than reimposing sanctions, Trump wants the Republican-controlled Congress to fix what he considers flaws in the agreement by including some "trigger points" that if crossed by Iran would automatically force US nuclear-related sanctions to go back into place.
Lawmakers would do this by amending the Iran Nuclear Agreement Review Act, a law Congress uses to monitor the nuclear accord, to strengthen nuclear inspections, cover Iran's ballistic missile program and eliminate the deal's "sunset clause" under which some of the restrictions on Iran's nuclear program expire over time.
"The deal is designed to prevent a nuclear weapon. It shouldn't have a sunset clause at all," said a senior administration official.
The ultimate goal is for a new agreement to be negotiated with Iran to cover its ballistic missile program and other US concerns, Tillerson said.
He said he had discussed with Iranian Foreign Minister Mohammad Javad Zarif at the United Nations in September the possibility of a second agreement alongside the 2015 accord that would address Iran's ballistic missile program and the sunset clauses.
"I indicated this to Foreign Minister Zarif when we saw each other on the margins of the U.N. I don't want to suggest to you that we give that a high chance of success but there is an openness to talk about it," Tillerson said.