U.S. President Donald Trump said he didn't think North Korean leader Kim Jong-un would break his promise on denuclearization, but conceded that he might.

Trump made the remarks Sunday aboard Air Force One returning to Washington from a two-week stay in Florida.

The U.S. president is believed to have sent a warning message to Kim not to break their 2018 Singapore summit deal after Kim threatened to abandon his self-declared moratorium on nuclear and long-range missile tests.

During the North's major ruling party meeting last week, Kim also said the world may soon witness a "new strategic weapon."

Trump's comments that Kim may break their promise is believed to be an indication that Washington is keeping tabs on a possible intercontinental ballistic missile test by Pyongyang.