President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Monday raised concerns that China may be seeking to undermine U.S. efforts to denuclearize North Korea but expressed confidence that leader Kim Jong Un will “honor the contract” he signed with the president.

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“I have confidence that Kim Jong Un will honor the contract we signed &, even more importantly, our handshake,” Trump tweeted. “We agreed to the denuclearization of North Korea. China, on the other hand, may be exerting negative pressure on a deal because of our posture on Chinese Trade-Hope Not!”

I have confidence that Kim Jong Un will honor the contract we signed &, even more importantly, our handshake. We agreed to the denuclearization of North Korea. China, on the other hand, may be exerting negative pressure on a deal because of our posture on Chinese Trade-Hope Not! — Donald J. Trump (@realDonaldTrump) July 9, 2018

The “contract” Trump referred to is a brief document he and Kim signed following their historic summit in Singapore, in which North Korea committed to “work toward the complete denuclearization of the Korean Peninsula.”

Less than a month later, doubts have grown that Pyongyang is willing to take concrete steps toward dismantling its nuclear arsenal.

North Korea accused the U.S. of pushing a “gangster-like demand for denuclearization” during a series of tense talks last week with Secretary of State Mike Pompeo Michael (Mike) Richard PompeoTreasury sanctions individuals, groups tied to Russian malign influence activities Navalny released from hospital after suspected poisoning Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers MORE.

Pompeo later dismissed those comments, saying the two sides “made progress” and that talks will continue.

“North Korea reaffirmed its commitment to complete denuclearization,” he said on Sunday during a visit to Japan. “We had detailed and substantive discussions about the next steps towards a fully verified and complete denuclearization.”

The talks came as the U.S. imposed billions of dollars in tariffs on Chinese imports, triggering fears of a full-blown trade war.

China is North Korea’s closest partner but had previously helped the U.S. exert pressure on Kim over his nuclear program.