South Korean President Moon Jae-in says he's ready for a fourth summit with North Korean leader Kim Jong Un to help salvage faltering nuclear negotiations between Washington and Pyongyang.

Moon's comments on Monday came after Kim over the weekend issued his harshest criticism yet over South Korea's diplomatic role, accusing Seoul of acting like an "overstepping mediator" and demanding that it diverge from Washington to support the North's position more strongly.

Moon met Kim three times last year and also brokered nuclear talks between Washington and Pyongyang following tensions created by North's nuclear and missile tests and the exchange of war threats between Kim and President Donald Trump.

The breakdown of a February summit between Kim and Trump has raised doubts about Moon's role as a diplomatic catalyst.