"We're still waiting for them. The possibility of another meeting between the two presidents obviously exists," said White House National Security Advisor John Bolton.

"But President Trump can't make the North Koreans walk through the door he's holding open. They are the ones that have to take the steps to denuclearize. And that's what we are waiting for."

Bolton said in a speech to the Federalist Society that in their Singapore meeting in June, Kim committed to getting rid of his nuclear weapons, and later agreed with South Korean President Moon Jae-in that it could be done in one year.

"If they would denuclearize, as they committed to do in Singapore, they could have a very different kind of life in North Korea," Bolton said.

On Friday Trump said e was expecting a "positive" new letter from Kim, indicating that negotiations remain alive after weeks of apparent deadlock.

"I know that a letter is being delivered to me, a personal letter from Kim Jong Un to me, that was handed at the border," Trump told reporters traveling with him to North Dakota.

"I think it's going to be a positive letter."