Secretary of State Mike Pompeo said Wednesday that the US would "soon" activate a controversial mechanism aimed at reimposing UN sanctions on Iran, despite opposition from its European allies.

The so-called "snapback" procedure, which President Donald Trump has said will be triggered this week, threatens to plunge the UN Security Council into crisis and has repercussions for the Iran nuclear deal.

"The president has made it clear we're going to do that soon and we will," Pompeo told reporters in Washington, declining to confirm a specific date.

The "snapback" aims to restore all international sanctions against Iran that were lifted as part of the 2015 accord with Tehran that sought to prevent it from developing nuclear weapons.

Trump withdrew the US from the agreement in 2018 and introduced American sanctions on Iran.

Despite pulling out of the deal, America claims that as a "participant" of the original agreement it has the power to unilaterally reimpose UN sanctions on Iran.

A 2015 UN Security Council resolution ratifying the agreement negotiated by former president Barack Obama says participating states can unilaterally reinstall sanctions if Iran has failed to significantly comply with the accord. 

The "snapback" procedure, which has never been used before, is supposed to lead to the re-establishment of sanctions after 30 days, without the possibility of Russia or China wielding their vetoes.

European countries on the Security Council contest the US's legal argument and fear that the return of sanctions will torpedo the nuclear deal. 

"This will be fully valid enforceable UN Security Council resolution. We have every expectation that it will be enforced just like every other UN Security Council resolution that is in place," Pompeo insisted.

Last week, the UN Security Council overwhelmingly rejected a US resolution to extend a conventional arms embargo on Iran.