The US Secretary of State said Iran would face further isolation and sanctions (Picture: PA).
The United States has joined the international condemnation of Iran's announcement that it is raising the level at which it enriches uranium - breaching the nuclear deal it struck with world powers in 2015.
Iran's move, coupled with earlier abandoning the deal's limit on its low-enriched uranium stockpile, intensifies pressure on Europe to find any effective way around US sanctions that block Tehran's oil sales abroad.
But the future of the accord that President Trump unilaterally pulled the US from a year ago remains in question.
While Iran's recent measures could be easily reversed, Europe has struggled to respond, even after getting a 60-day warning that the increase was coming.
Meanwhile, experts fear a miscalculation in the crisis could explode into open conflict, as President Trump already has nearly bombed Iran over Tehran shooting down a US military surveillance drone.
The US Secretary of State, Mike Pompeo, said in a tweet that Iran would face further isolation and sanctions:
"Nations should restore the longstanding standard of no enrichment for Iran’s nuclear program.
"Iran’s regime, armed with nuclear weapons, would pose an even greater danger to the world.
On Sunday, President Trump warned Tehran that "Iran better be careful".
International reaction to Iran's decision came swiftly, with Britain warning Iran to "immediately stop and reverse all activities" violating the deal, Germany saying it is "extremely concerned", and Israeli prime minister Benjamin Netanyahu, a longtime critic of the accord, urging world powers to impose so-called "snapback sanctions" on Tehran.
Japan expressed "serious concern" over Iran's decision and urged Tehran to return to its earlier commitment.