Donald Trump is expected to designate an elite wing of the Iranian military as a terrorist organisation next week as part of a new hardline strategy against the Islamic republic.

A senior administration official said the US president, who is poised to decertify a landmark nuclear deal between Iran and world powers next week, will not seek the reimposition of broad sanctions that would risk collapsing the 2015 accord.

But Mr Trump is expected to announce new measures against Iran, including the prospect of additional targeted sanctions, the designation of the Revolutionary Guard Corps as a terrorist organisation and the adoption of a tougher stance on Iranian proxies in Syria, Iraq and Yemen, according to a person briefed on the matter.

“It’s an integrated Iran strategy focused on neutralising and rolling back Iran’s malign activities regionally and globally,” the person said.

Hassan Rouhani defended the nuclear deal on Saturday. “We have achieved benefits that are irreversible. Nobody can roll them back, neither Trump, nor 10 other Trumps,” the Iranian president said.

US officials are frustrated by what they regard as Iran’s destabilising influence in Syria, Iraq and Yemen, as well as the reach of Lebanese militant group Hizbollah.

Iranian troops and Shia militias backed by Tehran, including Hizbollah, are fighting alongside regime forces in Syria’s six-year war. Hizbollah also has a political wing that is part of the Lebanese government.

“We must put an end to Iran’s continued aggression and nuclear ambitions,” Mr Trump said on Thursday before a meeting to discuss Iran with his senior military officials, adding it “supports terrorism and exports violence, bloodshed, and chaos across the Middle East”.

He later told reporters: “Maybe it’s the calm before the storm”, without making it clear if he was talking about Iran.

We must put an end to Iran’s continued aggression and nuclear ambitions

A person familiar with the new strategy said it would involve stepped-up covert action against Iran and its proxies. The Central Intelligence Agency declined to comment.

Mr Trump has until October 15 to announce whether he will recertify that Iran is complying with the 2015 nuclear deal, in which Tehran agreed to curb its atomic programme in exchange for limited sanctions relief.

Mr Trump has described the accord as the “worst deal ever” but critics say undermining it will raise tensions in the Middle East and make it more difficult to broker future agreements.

“If decertification is combined with massive escalation in the region — it doesn’t matter if he doesn’t push for sanctions,” said Trita Parsi, president of the National Iranian American Council and author of Losing an Enemy, which charts the history of the 2015 deal. “Where are the de-escalatory measures? There are none.

“Trump’s decision puts us back on the path to conflict with Iran, and that path could ultimately lead to war,” he said.

If he does not certify the nuclear agreement, Mr Trump would be handing the future of the deal to Congress, which would have 60 days to decide whether to reimpose nuclear-related sanctions. Even if Congress is persuaded not to do that, it is likely to try to develop a series of more punitive measures.

While only the Congressional leadership can initiate the reimposition of sanctions, the sudden focus on Iran may motivate Iran hawks such as Republican Senator Tom Cotton to initiate new legislation outside the deal.

The administration also hopes it can amend or extend existing legislation that requires Mr Trump to certify the deal at regular 90-day intervals. But Congressional aides say they are still waiting to see how Mr Trump will handle the re-certification deadline, and Congress is loath to give up oversight powers.

European signatories to the deal, to which Russia and China are also party, have spent weeks lobbying the administration and Congress against scrapping the agreement, which Mr Trump has described as the “worst deal ever”.

Sergei Lavrov, Russia's foreign minister, said on Friday the Iran deal should be "preserved to the fullest extent," following reports Mr Trump would not certify the deal.

European officials have concluded that most of the top US officials agree that the nuclear deal is worth preserving, and are trying to salvage the accord in a way that minimises problems for Mr Trump with his conservative base.

While many Republicans in Congress were critics of the Iran deal, the House and Senate leadership would rather not deal with the Iran situation now, particularly as they try to enact tax reform, a complex issue that has become their top priority for the remainder of the year.

Some Republicans are also nervous about how Mr Trump will handle the Iran situation, partly because he has on occasion made public statements on a range of issues that have undermined positions that they had already agreed with the president.

Additional reporting by Henry Foy in Moscow

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