US Secretary of State Mike Pompeo reiterated calls for North Korea to completely rid itself of all remnants of its nuclear weapons programme before international sanctions are lifted.
Speaking in a series of Friday interviews, Pompeo said that would include any possible clandestine sites unknown to those outside North Korea, reports CNN.
"I don't want to get too far into the details, but when you think about complete denuclearization, it would certainly be all of their sites, not just those that have been declared," Pompeo said.
"So we've got to make sure that it's complete," he added.
The denuclearization is a "great big commitment on the part of North Korea as well, and there'll be a parallel set of security assurances that are also big and bold and different", Pompeo said.
He suggested the administration would link those security assurances to the economic benefits that could stem from a deal.
The top US diplomat also held out a possibility that some sort of written statement or communique that lays out tangible achievements from the June 12 meeting between North Korean leader Kim Jong-un and American President Donald Trump could emerge.
"In the event that we are successful, as we are hopeful that we will be, yes, I would hope that there would be a statement that they would put out that each could agree to… But we'll have to see."
Pompeo will travel on to meetings with officials in South Korea and China after the summit in Singapore, CNN reported.
President Trump "is willing to do something big, something bold. I think that Chairman Kim Jong Un is prepared to do that as well", he said.