"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," he told reporters in Morristown, New Jersey.
Trump's top diplomat, Secretary of State Mike Pompeo, earlier Sunday said Iran will face further sanctions in response to the expected breach of the cap set by an endangered nuclear deal reached with international powers but from which the United States withdrew last year.
The 3.67 per cent enrichment limit set in the agreement is far below the more than 90 per cent level required for a nuclear warhead.
Iran had said Sunday it was set to breach the uranium enrichment cap set by an endangered nuclear deal within hours as it seeks to press other parties into keeping their side of the bargain.
The Rouhani-led government also threatened to abandon more commitments unless a solution is found with the remaining parties to the landmark 2015 agreement after Washington unilaterally pulled out.
US Secretary of State Mike Pompeo tweeted Sunday that Iran would face "further isolation and sanctions", adding to a raft of biting economic measures imposed by Washington, and President Donald Trump warned that Iran "better be careful."
Iran’s latest expansion of its nuclear program will lead to 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.
The 2015 deal was reached between Iran and six world powers - Britain, China, France, Germany, the United States and Russia - and saw Tehran agree to drastically scale down its nuclear programme in exchange for sanctions relief.
Iran had said it was set to breach the uranium enrichment cap set by an endangered nuclear deal within hours as it seeks to press other parties into keeping their side of the bargain.