United States President Donald Trump launched a tougher strategy to check Iran's “fanatical regime” on Friday and warned that a landmark international nuclear deal could be terminated at any time.

In a much-anticipated White House speech, Trump stopped short of withdrawing from the 2015 accord, but “decertified” his support for the agreement and left its fate in the hands of Congress.

And, outlining the results of a review of efforts to counter Tehran's “aggression” in a series of Middle East conflicts, Trump ordered tougher sanctions on Iran's Revolutionary Guards Corps and on its ballistic missile programme.

Trump said the agreement, which defenders say was only ever meant to curtail Iran's nuclear programme in return for sanctions relief, had failed to address Iranian subversion in its region and its illegal missile programme.

The US president said he supports efforts in Congress to work on new measures to address these threats without immediately torpedoing the broader deal.

“However, in the event, we are not able to reach a solution working with Congress and our allies, then the agreement will be terminated,” Trump said, in a televised address from the Diplomatic Room of the White House.

“It is under continuous review and our participation can be cancelled by me as president at any time,” he warned.

Simultaneously, the US Treasury said it had taken action against the Islamic Revolutionary Guards under a 2001 executive order to hit sources of terror funding and added four companies that allegedly support the group to its sanctions list.

Trump's criticism of the so-called Joint Comprehensive Plan of Action (JCPOA), the nuclear control accord reached between Iran and Britain, China, France, Germany, Russia and the United States, had raised global concerns.

World governments feared any US move to sabotage the arrangement could dash Washington's diplomatic credibility and relaunch Iran's alleged quest for a nuclear weapon, in turn provoking a new Middle East arms race.

But Secretary of State Rex Tillerson made clear ahead of the president's speech that his decision does not necessarily mean an end to the accord.

'The worst deal'

“The intent is that we will stay in the JCPOA, but the president is going to decertify,” Tillerson said.

“We're saying, fine, they're meeting the technical compliance,” he said, indicating that the broader agreement would remain intact for now, and that US lawmakers will have an opportunity to revisit the US sanctions regime.

Trump had repeatedly pledged to overturn one of his predecessor Barack Obama's crowning foreign policy achievements, deriding it as “the worst deal” and one agreed to out of “weakness.”

The agreement stalled Iran's nuclear programme and marginally thawed relations between Iran and what Tehran dubs the “Great Satan,” but opponents, and even some supporters, say it also prevented efforts to challenge Iranian influence across the Middle East.

But since coming to office, Trump has faced intense lobbying from international allies and much of his own national security team, who argue the JCPOA should remain in place.

Both the US government and UN nuclear inspectors say Iran is meeting the technical requirements of its side of the bargain, dramatically curtailing its nuclear programme in exchange for sanctions relief.

US concerns about the Guards could also weaken the deal. Trump stopped short of designating the powerful military faction a global terror organisation, as some hawks demanded, but his announcement of targeted sanctions is still likely to trigger an angry Iranian response.

Apart from running swaths of Iran's economy and Iran's ballistic program, the corps is also accused of guiding bellicose proxies from Hezbollah in Lebanon, to the Houthi in Yemen to Shia militia in Iraq and Syria.

“We have considered that there are particular risks and complexities to designating an entire army, so to speak, of a country,” Tillerson said.

Instead the US will squeeze those directly supporting the corps' “terrorist activities, whether it's weapons exports or it's weapons components, or cyber activity, or it's movement of weapons and fighters around.”

Snap back

Still, Trump's tough-guy approach could yet risk undoing years of careful diplomacy and increasing Middle East tensions.

Iranian President Hassan Rouhani lashed out at his US counterpart saying he was opposing “the whole world” by trying to abandon the landmark nuclear agreement.

“It will be absolutely clear which is the lawless government. It will be clear which country is respected by the nations of the world and global public opinion,” he added.

Congress must now decide whether to end the nuclear accord by “snapping back” sanctions, which Iran demanded be lifted in exchange for limiting uranium enrichment.

Trump will not ask Congress to do that, Tillerson said. “A re-imposition of the sanctions,” he said, “would, in effect, say we're walking away from the deal.”

But lawmakers may yet decide to torpedo the agreement.

Proposals by Republican Senators Tom Cotton and Bob Corker to introduce “trigger points” for new sanctions and extend sanctions beyond a pre-agreed deadline have spooked allies, who believe it could breach the accord.

But it remains unclear if their proposals can garner the 60 votes need to pass the Senate.

'Aggressive and threatening rhetoric'

Russia's foreign ministry criticised the new tougher Iran strategy announced by US President Donald Trump on Friday, accusing him of using “aggressive and threatening rhetoric”.

The ministry in a statement said that Moscow “underlines once again that it is unacceptable to use aggressive and threatening rhetoric” and that “using such methods to solve problems that touch fundamental security interests of other countries is bound to fail.”

It said Russia reacts “with sadness” to Trump's decision to withdraw his support for the 2015 Iran nuclear deal, but said it expected the accord to be honoured nonetheless.

“We view (the decision) as an element of a domestic debate in the framework of US national legislation,” it said.

“We expect that this step would not have a direct impact on the implementation of the deal.”

The Russian statement insisted that the nuclear deal “has already contributed to strengthening international peace and security”.

“Iran is strictly adhering to its responsibilities,” the foreign ministry added.

“Russia remains committed to the JCPOA, is interested in keeping it and will continue to carry out its obligations. We call on the other parties to do the same.”

Deputy Foreign Minister Sergei Ryabkov further told Interfax news agency that he finds the attitude of US lawmakers “rather worrying, I would even say bleak”.