US officials believe North Korea is posturing ahead of summit

But they expect the warnings to continue in the weeks ahead, even as both sides prepare for the historic diplomatic encounter. And they concede the development amounted to a reality check for Trump, who has enthusiastically promoted his upcoming dealings with Kim and embraced the potential for a major foreign policy victory.

If the summit is scrapped, American officials believe it will be by Kim and not by Trump. After being caught off guard by Tuesday's statement, which warned the talks could be at risk if the US continues to insist Pyongyang abandon its nuclear program, Trump's aides are now bracing for more of the same in the coming weeks and are prepared to ride it out.

Indeed, the rhetoric continued Thursday. Ri Son Gwon, North Korea's chairman of the Committee for the Peaceful Reunification, told a reporter from North Korean state media that improving relations with South Korea would remain stalled unless the two sides could resolve differences over joint US-South Korea military drills.

"It will not be easy to sit back with South Korea's current 'regime' unless the serious situation that suspended the high-level inter-Korean talks is resolved," he said. "The direction of future North-South relations will depend solely on the actions of the South Korean authorities."

Press conference pulled from schedule

It was the latest indication that the warmer tone emanating from North Korea had ended. So far, Trump has avoided matching the harsher language, as he did for most of last year as tensions ratcheted up between himself and Kim.

He has not tweeted on the latest developments, and was taciturn when questioned about the matter in the Oval Office on Wednesday.

"We'll have to see," he said.

Officials are hoping he'll remain largely silent to avoid inflaming the fragile diplomatic opening. A planned news conference with the secretary general of NATO was pulled from his schedule on Thursday, preventing him giving an extended answer on the topic.

His aides have offered similarly guarded responses. Asked during a photo opportunity Thursday whether the summit would still happen, Secretary of State Mike Pompeo did not answer. Pompeo has led diplomatic efforts with Kim, including traveling to North Korea twice for preparatory talks.

John Bolton, the President's national security adviser, conferred Wednesday morning with his South Korean counterpart, who offered little clarity on North Korea's intentions. He, along with other administration officials, said they believed the summit would still proceed as planned.

"We want to do whatever we can to make the meeting a success," Bolton told Fox News Radio. "But there should be no mistake that if we don't see that commitment to denuclearization then we're not going to make the mistakes of past administrations and fall into endless discussions with North Korea."

Even amid the uncertainty, aides were still proceeding as if the summit will happen. Teams of US officials are in Singapore surveying sites, and haven't been given any indication they should stop.