Trump and Mr moon of South Korea to hold talks on Kim summit
Mr Moon will try to calm US nerves to get talks back on track, observers say.
One of his officials said he would "likely tell President Trump what to expect and what not to expect from Kim", Yonhap news agency reported.
The summit was first thrown into doubt when North Korea cancelled high-level talks with South Korea last Wednesday, saying it was angered by Seoul's joint-military exercises with the US.
The North called the exercises a "provocation" and a rehearsal for an invasion.
Mr Trump later clarified that the US was not pursuing the "Libya model" for North Korea.
Ahead of the meeting with Mr Moon, US Vice-President Mike Pence warned Mr Kim not to "play" Mr Trump if they meet next month. Mr Pence said in a Fox News interview that such a move would be a "great mistake".
He also said there was "no question" that Mr Trump could walk away from the planned meeting in Singapore.
The planned summit first came about when Mr Trump accepted North Korea's invitation for direct talks earlier this year.
That followed more than a year of heated rhetoric between North Korea and the US, and global concern that the hostilities might escalate into military confrontation.
North Korea has conducted several nuclear tests over the past few years and has developed long-distance missiles it says can carry nuclear bombs as far as the US mainland.
If the summit happens, it would be historic as no sitting US president has ever met a North Korean leader.