"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 percent enrichment limit set in the agreement is far below the more than 90 percent level required for a nuclear warhead.