U.S. President Donald Trump on Sunday warned Iran to "be careful" after Tehran said it had started increasing its uranium enrichment beyond the 3.67 percent cap agreed in a 2015 nuclear deal with major powers.

Trump made the comment to reporters at an airport in New Jersey hours after Iran announced its decision to boost its uranium enrichment.

U.S. Secretary of State Mike Pompeo said in a tweet that Tehran's action would "lead to further isolation and sanctions," warning that "Iran's regime, armed with nuclear weapons, would pose an even greater danger to the world."

Iran's move came about a week after it said it had exceeded the limit on the amount of low-enrichment uranium in its stockpile under the accord.

Iran's 2015 nuclear deal with Britain, China, France, Germany, Russia and the United States promised sanctions relief to Tehran in exchange for curbs on its nuclear program, including limiting its stockpile to less than 300 kilograms of uranium enriched to a maximum of 3.67 percent.

Low-enriched uranium of between 3 to 5 percent can be used to make nuclear reactor fuel.

The United States withdrew from the Joint Comprehensive Plan of Action agreement in May last year and has imposed new sanctions on a variety of sectors covering automobiles, energy and financial transactions that have dealt a heavy blow to the Iranian economy.

On May 8, a year after the U.S. withdrawal, Iran announced it was suspending some of its commitments, while giving the five other countries remaining in the deal "60 days to fulfil their obligations" to provide Tehran with economic benefits under the accord.

Iran has warned it may further enrich uranium up to 20 percent, if leaders of the other countries in the deal fail to come up with the sanctions relief.

Uranium with that level of purity can be used at reactors for creating isotopes for cancer treatments. Experts say once 20 percent enrichment is reached, it is a relatively short step from reaching weapons-grade 90 percent enrichment.

The New York Times reported Iran's latest move is "the most threatening" among steps it has taken, while the Wall Street Journal reported Tehran has put the accord "at risk of collapse."