The report comes as talks between the U.S. and North Korea hit another snag last week, with a NY meeting between Secretary of State Michael Pompeo and the top negotiator from Pyongyang cancelled at the last minute.
In reports released by the Washington, D.C. -based Center for Strategic and International Studies, researcher Joseph Bermudez said maintenance and minor infrastructure improvements have been observed at some of the sites, despite the ongoing negotiations.
However, another South Korean analyst said that there is another way to convince Kim to make his promised visit to Seoul: "I think North Korea would want to come to Seoul on the back of some progress in its relations with the U.S. so that it could gain economic rewards - economic assistance, for example - from South Korea".
North Korea is conning the United States, building up secret, smaller missile bases even after publicly touting the dismantling of its main launch site, according to a new review of satellite images by a top think tank.
In a statement to The Times, the State Department said: "President Trump has made clear that should Chairman Kim follow through on his commitments, including complete denuclearization and the elimination of ballistic missile programs, a much brighter future lies ahead for North Korea and its people".
But the Trump administration says that sanctions must remain as pressure until a final accord, rejecting an incremental approach favored by ally South Korea as well as calls to end sanctions by China and Russian Federation.
Trump also reassured critics that talks with North Korean leader Kim Jong Un were still occurring.
Trump ruled out cemetery motorcade as disruptive
The visit was planned for the Aisne-Marne American Cemetery in France - where USA war dead were being honored - because of rain. Addressing one veteran, he joked: "You look so comfortable up there under shelter as we're getting drenched".
About 500 United States and South Korean marines began military drills last week that were among joint exercises indefinitely suspended in June as Seoul and Washington focused on engaging with North Korea.
North Korea has been playing hardball recently and that may indicate that the decision to resume testing is not far off.
"I realise I am a broken record, but North Korea has never offered to abandon its nuclear weapons", said Lewis. "I would love to take the sanctions off, but they have to be responsive, too", Trump said.
Trump has hailed his July summit with North Korean leader Kim Jong Un as having opened the way to denuclearization of the divided peninsula, defusing tensions that less than a year ago brought the two countries to the brink of conflict. "We're in no rush", he told reporters last week, when asked about the prospect of a second summit with Kim, adding that the rockets and missiles "have stopped".
The 248-kilometer (155-mile) border buffer peppered with millions of land mines has been the site of occasional skirmishes between the two forces since the 1950-53 Korean War. The "North Korea crisis" has been out of the news for a while, but one way or another expect the crisis to be in the news again soon, especially now that the midterms are over. They say there are seven more bases that remain hidden.
The bases are arranged in three belts across North Korea, according to the report, with those for strategic missiles deep inside the country.