US President Donald Trump will unveil a more aggressive strategy against Iran (AFP)

US President Donald Trump will unveil a more aggressive strategy against Iran on Friday but is expected to stop short of withdrawing from the landmark 2015 nuclear deal.

Officials say he will not kill the international accord outright, instead "decertifying" the agreement and leaving US lawmakers to decide its fate.

During a White House speech, Trump will declare the 2015 agreement - which curbed Iran's nuclear program in return for sanctions relief - is no longer in the US national interest.

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 strategy will seek to shield Israel from Iran's "unrelenting hostility" and counter the threat to all US interests and allies from Iran's proxy forces, ballistic missile development and eventual nuclear ambitions.

Iran Warns US Against Labeling Revolutionary Guards as Terrorists

German Foreign Minister Warns U.S. Against Abandoning Iran Nuclear Deal

But the plan as outlined by the fact sheet does not envisage Washington pulling out of the Iran deal's Joint Comprehensive Plan of Action or JCPOA.

Indeed, "the deal must be strictly enforced, and the IAEA must fully utilize its inspection authorities."

Since coming to office, Trump has faced intense lobbying from international allies and his own national security team, who argued the deal should remain in place.

In another partial climb-down, Trump is also expected to levy limited sanctions against the Revolutionary Guards, rather than invite retaliation by designating it as a terrorist organization.

The outcome "probably reflects more some of the divisions and debates within the administration," said former US Middle East envoy Dennis Ross.

Return to sanctions?

Trump's gambit could risk undoing years of careful diplomacy and increase Middle East tensions.

UN nuclear inspectors say Iran is meeting the technical requirements of its side of the bargain, dramatically curtailing its nuclear program in exchange for sanctions relief.

Iranian President Hassan Rouhani lashed out at his US counterpart, saying he was opposing "the whole world" by trying to abandon a 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.

Many lawmakers are waiting to see how Trump presents the choice before deciding whether to keep or torpedo the agreement.

In a statement to AFP, leading Republican Senator Marco Rubio described the accord as "fatally-flawed" and said he was open to legislation that would "substantially improve America's ability to counter Iran's nuclear, terrorism, militancy and regional threats."

Allies warn against ditching deal

Trump has been railing against the Iran deal since before he was elected in November last year.

In office, he has chafed at being required under US law to re-certify Iran's compliance with the accord every 90 days, declaring that Tehran has broken it "in spirit."

The other signatories to the deal have urged Washington not to let it fall apart.

The Kremlin warned Washington on Friday that abandoning the Iran nuclear deal would be a heavy blow to international relations and non-proliferation efforts.

"This could seriously aggravate the situation around the Iranian nuclear dossier," President Vladimir Putin's spokesman Dmitry Peskov told journalists.

Read more: From Iran to UNESCO: Trump's Withdrawal Doctrine

"Such actions will unequivocally damage the atmosphere of predictability, security, stability and non-proliferation in the entire world."

China also issued a statement which called on all parties to "preserve and implement this deal."

"We believe this deal is important to ensuring the international nuclear nonproliferation regime and regional peace and stability", foreign ministry spokeswoman Hua Chunying said.

Secretary of State Rex Tillerson spent much of the week on the telephone, talking through a decision that is deeply unpopular with allies.

Europe fears not only that Iran will resume the quest for the bomb but that the US is relinquishing its leadership role in a stable, rules-based international system.

Separately, Russian Foreign Minister Sergei Lavrov, speaking to Iranian Foreign Minister Mohammad Javad Zarif by phone, expressed Moscow's "full commitment" to the Iran nuclear deal, known as the Joint Comprehensive Plan of Action, the foreign ministry said on Friday.

The agreement was signed between Iran and six world powers - Britain, China, France, Germany, Russia and the US - at talks coordinated by the European Union.