President Donald Trump on Friday cited "an unusual and extraordinary threat" from North Korea's nuclear arsenal to extend sanctions on Kim Jong-un's regime, despite touting the success of a historic summit earlier this month.

After flying back to Washington last week, boasting of success, the US leader tweeted: "There is no longer a Nuclear Threat from North Korea."

"Sleep well tonight!" he added on June 13, a day after the Singapore meeting.

But a presidential declaration sent to Congress on Friday struck a different note as it explained why the administration would keep in place tough economic restrictions first imposed by former president George W Bush.

"The existence and risk of proliferation of weapons-usable fissile material on the Korean Peninsula and the actions and policies of the Government of North Korea continue to pose an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States," it said.