"Since 2018, chairman Kim has written eight letters to President Trump, and President Trump has written four letters to chairman Kim," Kim Yeon-chul, the South's Unification Minister in charge of inter-Korean affairs, told reporters.

"I guess those leaders recognise the importance of resuming the dialogue between the two countries as they are constantly exchanging letters."

Seoul is set to host the US President this weekend as it tries to rekindle nuclear talks that have stalled since the collapse of a second summit between Trump and Kim in Hanoi in February.

Minister Kim's comments Wednesday came hours after the South's President Moon Jae-in said Pyongyang and Washington were engaged in "behind-the-scenes talks" for another summit and noted the most recent exchange of letters between Trump and Kim.

On Sunday, the North's state media said Kim received a personal message "of excellent content" from the US president, just days after Trump said Kim had sent him a "beautiful letter".

The US president's trip to Seoul comes a week after Chinese leader Xi Jinping made a highly symbolic visit to Pyongyang.

Xi is expected to meet Trump at the G20 summit in Japan this weekend and analysts say his trip to the North was intended to signal to Trump his influence with Kim.

Minister Kim said there was "strategic competition" between China and the US on the issue of the North's nuclear arsenal, adding Beijing, Washington and Seoul needed to cooperate.