"I'm concerned South Korea's visit is going to undermine efforts by @SecPompeo and Ambassador @nikkihaley to impose maximum pressure on the North Korean regime," he wrote on Twitter, using the hashtags for US Secretary of State Mike Pompeo and US.

Ambassador to the United Nations Nikki Haley. "While North Korea has stopped testing missiles and nuclear devices, they have NOT moved toward denuclearization."

The administration of US President Donald Trump has led an international campaign to pressure North Korea to give up its nuclear weapons program through sanctions.

Kim committed to work toward the "complete denuclearization" of the Korean Peninsula at both his previous meetings with Moon and at his historic Singapore summit with Trump in June.

But negotiations between Washington and Pyongyang have stalled, causing some in the US to question the North Korean leader's sincerity.

"South Korea should not be played by Kim Jong Un," Graham wrote in a separate tweet. (Yonhap)