The official declaration came in a notice to Congress on Friday, in which Trump ordered that economic restrictions would continue for one year, The Washington Post reported.
The national emergencyTrump signed will keepin place sanctions first imposed a decade ago by President George W. Bush.
It also allows Washington to forbid North Korean leaders from selling or using any assets they may hold in the United States. It is separate from US sanctions related to the Norths human rights issues and international penalties imposed over its nuclear and ballistic missile programs.
"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," Trump wrote on Friday.
The paperwork came a day after Trump said at a Cabinet meeting in the White House that North Korea had already started a process of total denuclearization, adding that Pyongyang had blown up four of its big test sites.
However, US officials familiar with current intelligence on North Koreas nuclear and missile test sites said there was no such evidence.
Trumps stern tone and list of accusations against North Korea marked a complete reversal from Trumps optimistic language following his June 12 summit with Kim in Singapore.
"Sleep well tonight!" the president tweeted on June 13. "There is no longer a Nuclear Threat from North Korea."
Following their meeting, the first between a sitting US president and a North Korean leader, Trump and Kim signed a joined document, committing to establishing new relations and achieving peace on the Korean Peninsula.
Before signing the document, Kim said the two leaders had decided to leave the past behind and that the world will see a major change.
Trump said he had formed a very special bond with Kim and that Washingtons relationship with Pyongyang would be very different.
While the summit was seen as a test for diplomacy that could end the long-running nuclear standoff, foreign policy experts have warned that the stakes are still high for an armed conflict if diplomacy fails.
The US seeks the complete and irreversible dismantling of North Koreas nuclear program. Pyongyang is demanding a solid guarantee of its security and the removal of Washingtons nuclear umbrella protecting allies South Korea and Japan.
