Trump wrote: "We are a long way from conclusion on North Korea, maybe things will work out, and maybe they won't - only time will tell."
In fact, Kim had not included a promise to rid his country of existing nuclear weapons, and analysts remained doubtful he would give up the missiles he believed guaranteed his regime's survival.
Trump's comments reportedly surprised White House officials, who had expected some kind of confidence-building words before the planned summit.
They expressed their scepticism to the Washington Post, suggesting Kim was offering modest pledges to create the "illusion" that he was open to negotiation but which could easily be reversed.
Regional experts suggested Trump would do well to point out that North Korea was unlikely to give up its weapons after spending decades developing a deterrent.
Nam Sung Wook, a professor of North Korean Studies at Korea University in Seoul, said: "North Korea has a long history of raising the issue of denuclearisation and has committed to freeze its nuclear weapons programmes in the past.
"We all remember how those pledges and commitments went down over past decades."
However, the current rapprochement still marks a stark change from last year, when the two leaders swapped insults and threatened war.
The Wall Street Journal reported today that Trump will ask Kim to dismantle the country's nukes without conceding ground on sanctions.
"When the President says that he will not make the mistakes of the past, that means the US will not be making substantial concessions, such as lifting sanctions, until North Korea has substantially dismantled its nuclear programmes," a senior Trump Administration official told the newspaper.
