Trump made the remarks as Iran moved to increase uranium enrichment beyond the purity threshold reached in the 2015 nuclear agreement between Tehran and six major powers -- the United States, Britain, China, France, Germany and Russia. Washington pulled out of the deal last year.
Iran on Monday warned its next step in reducing commitments under the Joint Comprehensive Plan of Action (JCPOA) will be stronger, with a senior nuclear official saying that 20% uranium enrichment is an option.
Spokesman for Iran's Atomic Energy Organization Behrouz Kamalvandi said Tehran has passed the 3.67% uranium enrichment cap set by a 2015 nuclear deal reached about 4.5%.
In response, Trump said that "Iran better be careful."
"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.
"Iran is doing a lot of bad things," Trump said. "The way they want it they would have automatic rights to have nuclear weapons. Iran will never have a nuclear weapon."
Trump has said that he would be Iran's "best friend" and that the Islamic republic could be a "wealthy" country if it stopped pursuing nuclear weapons. Iran’s leadership has repeatedly expressed their firm opposition to the development and use of nuclear weapons.
Commenting on Trump’s statement, American political analyst and activist Myles Hoenig said, “It is hard to get rid of something (nuclear weapons) you never had to begin with.”
The US withdrew from the nuclear deal in May 2018 and began reimposing sanctions on Iran in August 2018, targeting crucial sectors including oil exports and the banking system.
In a meeting with Japanese Prime Minister Shinzo Abe last month in Tehran, Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei said, “I have issued a fatwa (religious decree) prohibiting the production of nuclear weapons, but you need to know that if we were after building nuclear arms, the United States could not do anything about it.”
The Leader emphasized that Washington is in no position to decide which country can be in possession of nuclear arms while it is, itself, stockpiling several thousands of nuclear warheads.