Imagery of the Sakkanmol missile base "continues to show minor infrastructure changes to the base that are consistent with what is often seen at remote [Korean People's Army] bases of all types", the report said.
South Korea has attempted to play down the significance of the undeclared missile sites. "We have been watching this site and there has been nothing new from this", said Kim Eui-kyeom, a presidential spokesman said in a press briefing on Tuesday.
Located 135 kilometers north of the South Korean capital of Seoul, it can reach the city in a short period of time, it said.
Asked about the report, Trump's national security adviser John Bolton told reporters in Singapore on November 13 that Trump "has given North Korea an incredible opportunity to walk through a door to a different future if they denuclearize ... but they still need to do that". "Kim Jong Un only committed voluntarily to halt long-range missile tests".
North Korea declared its nuclear force "complete" and halted missile and nuclear bomb testing earlier this year but USA and South Korean negotiators have yet to elicit from Pyongyang a concrete declaration of the size or scope of the weapons programs, or a promise to stop deploying its existing arsenal.
Since the summit in Singapore, North Korea has forgone nuclear and missile tests, dismantled a missile test site and promised to also break up the country's main nuclear complex if the USA makes concessions.
Before we set off too many alarm bells here, it's worth noting that none of these are new nuclear sites that the North Koreans have been working on during the negotiations.
"Trump would then declare victory, say he got more than any other American president ever got, and the threat would still be there".
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Construction began on the Sakkanmol Missile Operating Base between 1991 and 1993 and it is thought to have been completed in 2001, and added to since 2011 when Mr Kim came to power.
A State Department official, asked whether those hidden sites went against the spirit of the summit and whether North Korea must give them up, said: "Donald Trump had made clear that should Chairman Kim follow through on his commitments - including complete denuclearisation and the elimination of ballistic missile programmes - a much brighter future lies ahead for North Korea and its people".
"Rather than returning to an operating base - which will undoubtedly be the target of repeated attacks - both the technical support element and launchers will remain in the field using pre-positioned reloads and supplies while moving frequently to different pre-surveyed locations", the report said.
US officials have said sanctions forced North Korea to the negotiating table and vowed to keep pressure until complete denuclearization. "It has never signed any agreement, any negotiation that makes shutting down missile bases mandatory".
"Any missile at these bases can take a nuclear warhead", the report's co-author, Joseph Bermudez, an authority on North Korea, told the Times.
"In short, from the CSIS report we can infer that first, North Korea is not honest enough with negotiating and second, there's no change in their nuclear capacity", he said.
The Trump administration abandoned Victor Cha as its pick for the South Korean ambassador's post because Cha disagreed with the "bloody nose" preventive strike strategy proposed by some in the administration as a way to achieve the president's goal of removing nuclear weapons from the Korean peninsula, according to analysts interviewed by the South China Morning Post at the time.
US Secretary of State Mike Pompeo had been scheduled to meet Kim's right-hand man, Kim Yong Chol, in NY last week to discuss denuclearization efforts and prepare for a possible second summit, according to the State Department.