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.

"I am continuing for one year the national emergency with respect to North Korea," added the statement.

Though the notice is considered pro forma, the disparity in tone reflects the work that US officials concede remains to be done as negotiators thrash out the details of Pyongyang's disarmament.

At their summit, Kim and Trump signed a pledge "to work towards complete denuclearization of the Korean Peninsula," a stock phrase favored by Pyongyang that stopped short of longstanding US demands for North Korea to give up its atomic arsenal in a "verifiable" and "irreversible" way.

Critics have pointed to the vague wording of the non-binding summit document and raised fears that the summit could weaken the international coalition against the North's nuclear program.