Setting the stakes sky high, Mr Moon said, "The fate and the future of the Korean Peninsula hinge" on the meeting.
The summit, planned for Singapore, offers a historic chance for peace on the peninsula — but also the risk of an epic diplomatic failure that would allow the North to revive and advance its nuclear weapons programme.
Mr Trump's newfound hesitation appeared to reflect recent setbacks in efforts to bring about reconciliation between the two Koreas, as well as concern whether the self-proclaimed dealmaker can deliver a nuclear accord with the North's Kim Jong Un.
In an extraordinary public airing of growing uncertainty, Mr Trump said "there's a very substantial chance" the meeting will not happen as scheduled.
Seated in the Oval Office with Mr Moon, Mr Trump said Mr Kim had not met unspecified "conditions" for the summit.
However, the president also said he believed Mr Kim was "serious" about negotiations, and Mr Moon expressed "every confidence" in Mr Trump's ability to hold the summit and bring about peace.
President Trump hosted President @moonriver365 at the White House today for a productive bilateral meeting and working lunch.