U.S. President Donald Trump sent his congratulations on the birthday of the late North Korean founding leader Kim Il-sung earlier this week, according to U.S. National Security Adviser John Bolton.

The message was sent as part of Trump's efforts to maintain good relations with Kim's grandson and current North Korean leader, Kim Jong-un, as they try to negotiate the dismantlement of the regime's nuclear weapons program, Bolton said Wednesday in an interview with U.S. broadcaster PBS.

Monday marked the founder's 107th birthday, a major national holiday in the North.

"I don't know how President Trump can be more forthcoming in his efforts to have a good relationship with Kim Jong-un," Bolton said. "He sends him pictures. He sends him letters. Happy birthday to Kim Jong-un's grandfather on his birthday on April the 15th."

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