Donald Trump has said his planned summit with North Korean leader Kim Jong Un “may not work out for June 12” and could be delayed.
Mr Trump is talking with South Korean President Moon Jae-in at the White House for consultations ahead of the summit in Singapore.
He said he does not like the change in attitude he has picked up from Mr Kim, and that he detected a shift after the North Korean leader and Chinese President Xi Jinping met for a second time in China.
Mr Trump said he hopes China is not influencing Mr Kim.
The White House meeting is happening as efforts to build peace between the two Koreas have hit a setback.
North Korea pulled out of planned peace talks with South Korea last week, objecting to long-scheduled joint military exercises between the US and the Republic of Korea forces.
North Korea has also threatened to abandon the planned Trump-Kim meeting over the US insistence on denuclearising the Korean Peninsula.
Mr Moon opened the talks with Mr Trump saying that the “fate and the future” of the Korean Peninsula hinges on the Singapore summit.
He said he believes Mr Kim is “serious” about the talks and urged him to “seize this opportunity”.
Earlier, Mike Pence said no concessions have been offered to North Korea in the run-up to the summit.
The US vice president told Fox News Radio that it would be a “mistake” for the North Koreans to think they can “play” Mr Trump.
Mr Pence said both the Clinton and Bush administrations had been “played” by the North Korean government.
“We offered concessions to the North Korean regime in exchange for promises to end their nuclear weapons programme, only to see them break those promises and abandon them,” he said.
He said the US remains open to the summit, and noted that the invitation to meet originated with the North Korean leader.