Iran made the expected move last week, putting it out of compliance with the 2015 deal originally agreed under then-US President Barack Obama and five other nations. Tehran had earlier warned it would increase uranium enrichment if the deal's European signatories did not help ease sanctions on its banking and oil sectors.

"Iran better be careful," Trump said on Sunday.

"Because you enrich for one reason and I won't tell you what the reason is. But it's no good; they better be careful," he said before boarding Air Force One in New Jersey.

Tehran has long maintained that it is not developing a nuclear weapons program and its enrichment and research is focused on non-military uses.

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