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.

- More drills shelved -

Also Friday, the US and South Korea agreed to indefinitely suspend two exchange program training exercises, to support diplomatic negotiations with North Korea, the Pentagon said.

The move came after the two countries had previously announced the shelving of the large-scale Ulchi Freedom Guardian exercises slated for August, making good on a pledge by Trump during his summit.

The decision followed a meeting between Defense Secretary Jim Mattis, Secretary of State Mike Pompeo, Chairman of the Joint Chiefs of Staff Joe Dunford, and National Security Advisor John Bolton.

"To support implementing the outcomes of the Singapore Summit, and in coordination with our Republic of Korea ally, Secretary Mattis has indefinitely suspended select exercises," Pentagon spokeswoman Dana White said in a statement.

Two Korean Marine Exchange Program training exercises scheduled to occur in the next three months have now been shelved.

US and South Korean forces have been training together for years, and routinely rehearse everything from beach landings to an invasion from the North, or even "decapitation" strikes targeting the North Korean regime.

Pyongyang typically reacts furiously. Following drills last year, the North fired ballistic missiles over Japan, triggering global alarm.