Iran's uranium enrichment level passed 4.5 percent on Monday, exceeding the limit set by the 2015 six-nation nuclear deal.
Iran announced on Sunday it would no longer adhere to the enrichment limit in what it billed as Teheran's second step to decrease commitments to the deal in a bid to press other parties into keeping their side of the bargain.
US President Donald Trump withdrew from the agreement between Iran and six world powers in May 2018 and has since reimposed sanctions on many sectors including the crucial oil and financial industries.
Iran wants the other parties -- France, Germany, Britain, China and Russia -- to take steps to guarantee the economic benefits Iran was promised for the drastic limitations imposed on its nuclear programme.
However after one year of what it called "strategic patience", Tehran has grown increasingly frustrated about a perceived lack of action by the European side to help it economically in the face of crippling US sanctions.
Threat of further US sanctions
US Secretary of State Mike Pompeo, on Sunday said Iran will face further sanctions in response to the expected breach of the cap set by an nuclear deal reached with international powers in 2015 but from which the United States withdrew last year.
The 3.67 percent enrichment limit set in the agreement is far below the more than 90 percent level required for a nuclear warhead.
"Iran better be careful, because you enrich for one reason, and I won't tell you what that reason is. But it's no good. They better be careful," the US President told reporters in Morristown, New Jersey.
The European Union said on Monday it was "extremely concerned" by Iranian plans to breach the uranium enrichment cap set by the 2015 nuclear deal, calling on Tehran to reverse course.
"We strongly urge Iran to stop and reverse all activities that are inconsistent with the commitments made under the Joint Comprehensive Plan of Action," said an EU statement.
Russia has expressed concern at the latest Iranian decision, hoping that diplomatic discussions can resolve the stand-off provoked by the United States.