U.S. President Donald Trump reportedly sent his congratulations for the birthday of the late founding leader of North Korea Kim Il-sung earlier this week.

During an interview with U.S. broadcaster PBS on Wednesday, U.S. National Security Adviser John Bolton explained how Trump is working to maintain good relations with North Korean leader Kim Jong-un.

Bolton said that Trump sends the North Korean leader pictures and letters, and also wished a happy birthday to the leader's grandfather. 

Monday marked Kim Il-sung's 107th birthday, a major national holiday in the North. 

Bolton did not elaborate on whether Trump sent a simple congratulatory note or a particular message regarding bilateral nuclear negotiations, which have remained deadlocked since the collapse of the second Trump-Kim summit in Hanoi in February.