The EU has strongly urged Iran to reverse its decision to raise the level at which it enriches uranium beyond that allowed by a 2015 nuclear deal.
Spokeswoman Maja Kocijancic said the bloc was extremely concerned after Iran announced that it was producing material with a fissile purity of 4.5%.
European leaders fear it could speed up the potential development of a nuclear weapon - something Iran denies seeking.
Iran has said it is responding to sanctions the US reinstated last year.
The country has also accused European powers of failing to protect its economy from the sanctions' effects and insisted that its actions are in line with the deal - known formally as the Joint Comprehensive Plan of Action or JCPOA.
It has promised to keep reducing its commitments every 60 days unless it begins to see the accord's promised benefits, and warned the Europeans that "certain strange acts" will prompt it to "skip all the next steps" and implement the final one.
The five other parties to the deal - the UK, France, Germany, Russia and China - could refer the issue to a "joint commission", a process that could ultimately lead to the "snap back" of the UN and multilateral sanctions lifted three years ago.
"We are extremely concerned at Iran's announcement that it has started uranium enrichment above the limit of 3.67%," Ms Kocijancic told reporters on Monday.
"We strongly urge Iran to stop and reverse all activities inconsistent with its commitments... We are in contact with the other JCPOA participants regarding the next steps under the terms of the JCPOA, including a joint commission," she said.
Chinese foreign ministry spokesman Geng Shuang said the "root cause" of the crisis was "the maximum pressure exerted by the US on Iran".
US Vice-President Mike Pence said the pressure campaign would "change Iran's malign behaviour and hold the regime accountable for its destructive actions", and vowed that the US would "never allow Iran to obtain a nuclear weapon".
