He said the second meeting between US President Donald Trump and North Korean leader Kim Jong-un will take place early next year, with the date and location still to be confirmed.

"I think it will be absolutely imperative in this next summit that we come away with a plan for identifying all of the weapons in question, identifying all the development sites, allowing for inspections of the sites and the plan for dismantling nuclear weapons," Pence said, noting that it is time to "see results."

Trump and Kim held their first summit in Singapore in June and agreed to work toward "complete" denuclearization of the Korean Peninsula in exchange for security guarantees from the US.

Details of the agreement have yet to be worked out. And last week's planned New York meeting between US Secretary of State Mike Pompeo and his North Korean counterpart was abruptly postponed over what the US called scheduling issues.

"Until we have a plan that is implemented to achieve complete, verifiable, irreversible, denuclearization, we're going to keep the pressure on," Pence said, referring to sanctions on the regime.

North Korea has increasingly called for sanctions relief while the US has insisted that denuclearization come first.

Meanwhile, the Center for Strategic and International Studies, a US think tank, reported this week that it has identified 13 of an estimated 20 North Korean missile operating bases that remain undeclared. (Yonhap)