President Donald Trump says the planned Singapore summit with North Korea's Kim Jong Un could be delayed.
The event had been planned for 12 June but there have been concerns that Mr Kim is not committed to denuclearisation.
The doubts about the session were raised in talks between Mr Trump and South Korean President Moon Jae-in, who came to Washington to urge the US president not to let a rare opportunity with reclusive North Korea get away.
As he addressed the media about his meeting with Mr Moon, he said: "North Korea in this case is the big one.
"The big topic will be Singapore and the meetings. We'll have to see what happens and whether or not it happens.
"If it does, that'll be great. It'll be great, I think, for North Korea. If it doesn't, that's okay too. I think a lot of good things will happen."
Mr Trump added: "If it doesn't happen, maybe it will happen later. It may not work out for 12 June."
The warning came as a UN monitoring group moved to enlist the help of the world's biggest oil trading companies to enforce sanctions against the rogue state.
There is a cap on the amount of oil North Korea can import and the coordinator of a panel of experts appointed by the UN Security Council said they were seeking support from the top traders to help implement sanctions.
Mr Trump said he did not like a change in attitude he had picked up from the North Korean leader.
He said he detected a shift after Mr Kim and Chinese President Xi Jinping met for a second time in China.
The US president said he hoped China had not influenced Mr Kim.
Mr Moon earlier said the "fate and the future" of the Korean peninsula hinges on the upcoming summit.
The South Korean president said he believed they were "one step closer" to the dream of a denuclearised Korean peninsula.
1:36Mr Moon said Mr Kim was "serious" about the talks.
Sky News is the only British broadcaster that has been invited to see North Korea's Punggye-ri nuclear site as it is dismantled.
Mr Kim has been keen to show the world he is carrying out his promise to denuclearise North Korea before the summit.
Sky's Tom Cheshire said on Tuesday the group of journalists would have to take a 12-hour train ride into the mountains, followed by a four-hour bus journey and a two-hour hike to Punggye-ri.
Mr Trump said last week "perhaps Kim Jong Un doesn't want a summit" as he refused to back down from military drills in the Korean peninsula.
He added that the "best thing" the North Korean leader could "ever do" was make a deal.
2:17Pentagon spokeswoman Dana White said there were no plans to change or reduce the scope of the joint Max Thunder drills between the US and the South.
She said the exercises were long-planned, were defensive in nature and were meant to ensure the readiness of US and South Korean forces.
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Exercise Max Thunder finishes on 25 May and includes aircraft from across the US military services.
Last Wednesday, North Korea cancelled a second round of talks with South Korea with hours to go in retaliation over the drills.
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