South Korean President Moon Jae-In delivered a message to President Trump on behalf of North Korea that said dictator Kim Jong Un is "determined" to denuclearize the peninsula, a South Korean diplomat said Tuesday.

"Moon Jae-In briefed the United States about the outcome of the inter-Korean summit and handed Chairman Kim Jong Un’s message to President Trump,” Ambassador Woo Yoon-keun told Russian media outlets. "He made it clear that Kim Jong Un was determined to ensure the denuclearization of the Korean Peninsula."

[Opinion: If Trump’s romance with Kim Jong Un were a rom-com]

Kim’s correspondence with Trump has been a key element of his diplomacy with the U.S. The missives provide a counterpoint to North Korea’s recalcitrance with Secretary of State Mike Pompeo, who has been trying to maintain pressure on the regime while offering the prospect of economic relief if Kim dismantles his nuclear weapons and ballistic missile programs.

The letters have made an impression on the president. "I was really being tough, and so was he,” Trump said Saturday. “And we would go back and forth. And then, we fell in love. No really. He wrote me beautiful letters. They were great letters. And then, we fell in love.”

Moon delivered Kim's latest message at the United Nations General Assembly last week. In public, North Korea took a defiant posture, issuing another demand that the U.S. lift sanctions in advance of denuclearization.

"The perception that sanctions can bring us on our knees is a pipe dream of the people who are ignorant of us," North Korean Foreign Minister Ri Yong Ho said in a speech to the UNGA on Saturday. "Without any trust in the U.S., there will be no confidence in our national security, and under such circumstances there is no way we will unilaterally disarm ourselves first.”

That speech came on the heels of China and Russia issuing a call for Western powers to ease at least some sanctions as a reward for North Korea’s recent behavior. “Negotiations are a two-way street,” Russian Foreign Minister Sergey Lavrov told the United Nations Security Council last week. “Steps by the DPRK toward gradual disarmament should be followed by easing of sanctions.”

The U.S. has accused Russia of systematically working to undermine sanctions on North Korea. And the Treasury Department has imposed sanctions on some Russian and Chinese entities complicit in those violations.

“We must not forget what’s brought us this far: the historic international pressure campaign that this council has made possible through the sanctions that it imposed,” Pompeo told the Security Council. "Until the final denuclearization of the DPRK is achieved and fully verified, it is our solemn collective responsibility to fully implement all U.N. Security Council resolutions pertaining to North Korea.”