US President Donald Trump has suggested that a planned historic meeting with North Korea's Kim Jong Un could be delayed.
"There's a very substantial chance that it won't work out" for June 12, he said.
Mr Trump raised the possibility that the meeting could be pushed back during a White House meeting with South Korea President Moon Jae-in, trying to co-ordinate strategy as concerns mounted over ensuring a successful outcome for the North Korea summit.
He told reporters: "If it doesn't happen, maybe it happens later," reflecting recent setbacks in efforts to bring about reconciliation between the two Koreas.
The North pulled out of planned peace talks with the South last week, objecting to long-scheduled joint military exercises between US and Republic of Korea forces.
And the North threatened to abandon the planned Trump-Kim meeting over US insistence on denuclearising the peninsula.
"There are certain conditions that we want," Mr Trump said on Tuesday.
He added that if they were not met, "we won't have the meeting".
He declined to elaborate on those conditions.
Mr Trump said "there's a very substantial chance" that the meeting will not take place on June 12.
"That doesn't mean it won't work out over a period of time," he said.
"But it may not work out for June 12. But there is a good chance that we'll have the meeting."
Mr Moon said in the Oval Office that the "fate and the future" of the Korean Peninsula hinged on the talks, telling the US president that they were "one step closer" to the dream of a denuclearised Korean Peninsula.
Mr Trump said he had noticed "a little change" in Kim Jong Un's "attitude" after Kim took a second trip to China this month in the run-up to the summit.
"I don't like that," Mr Trump said.
Mr Trump said he hoped that Chinese President Xi Jinping was committed to the goal of denuclearising the Korean peninsula, calling him a world-class poker player.
But he said he was displeased by China's softening of border enforcement measures against North Korea.
Mr Trump encouraged Mr Kim to seize the opportunity for the meeting and to make a deal to abandon his nuclear programme, pledging not only to guarantee Mr Kim's personal security, but also predicting an economic revitalisation for the North.
"I will guarantee his safety, yes," Mr Trump said, if Mr Kim agreed to complete, verifiable, and irreversible denuclearisation.
He said if an agreement was reached, China, Japan and South Korea would invest large sums to "make North Korea great".