The US president and North Korean strongman met in Singapore almost a month ago for an unprecedented summit between sitting leaders of the two countries, which resulted in a general joint declaration that did not include a timetable or concrete commitments from North Korea on denuclearization.

"I have confidence that Kim Jong Un will honor the contract we signed &, even more importantly, our handshake," Trump tweeted, emphasizing the importance he places on personal relationships.

"We agreed to the denuclearization of North Korea," he wrote, despite the fact that the declaration signed by the two leaders refers to the denuclearization of the "Korean peninsula" -- a vaguer formula that leaves room for various interpretations.

Trump also expressed doubts about Beijing's attitude toward the negotiations between Washington and Pyongyang, suggesting that China could play a counterproductive role due the trade war that has seen the world's two largest economies slap tariffs on each other's goods.

"China, on the other hand, may be exerting negative pressure on a deal because of our posture on Chinese Trade-Hope Not!" Trump wrote.

Despite Trump's confidence that Kim is committed to giving up nuclear weapons, the reconciliation process between the US and North Korea was thrown into crisis on Sunday when Pyongyang angrily rejected Washington's "unilateral and gangster-like" demand for rapid nuclear disarmament.

The North's foreign ministry took exception to Secretary of State Mike Pompeo's effort to secure concrete commitments to the promise Kim made at the summit with Trump.

The tone was in stark comparison to Pompeo's characterization of the talks he held in Pyongyang as a success, though critically he failed to present any new details as to how North Korea would honor its summit commitment to "denuclearize" in exchange for US security guarantees.

Speaking privately, US officials suggested the harshly-worded North Korean statement was a negotiating tactic.

Pompeo reiterated in Tokyo on Sunday that sanctions on North Korea will remain in place until Pyongyang achieves "final" denuclearization.