North Korean leader Kim Jong Un and U.S. President Donald Trump pledged to work toward denuclearization at their landmark June summit in Singapore but the agreement was short on specifics and negotiations have made little headway.
"I will be the first to let you know if things go bad!" he wrote.
"North Korea's decommissioning of the Sohae satellite launch facility, while gaining much media attention, obscures the military threat to US forces and South Korea from this and other undeclared ballistic missile bases", it said.
President insists the U.S.is fully aware of all of North Korea's missile sites amid reports that new satellite images reveal "hidden" bases.
"We have known for some time that the North Koreans are starting to deploy and even test missiles at the unit level", Stephan Haggard, a professor of Korea-Pacific studies at the University of California at San Diego said.
The U.S. State Department did not comment on whether activity in the reported missile sites would constitute a breach of Kim's agreement with Trump on denuclearization.
Yet a Cheong Wa Dae spokesman responded to the report about the secret missile sites by saying it contains "nothing new" and that the North never promised to destroy any missile sites other than Tongchang-ri.
Photos by Beyond Parallel/CSIS show possible missile operating bases in North Korea.
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Bases in the furthest, or "strategic", belt are more than 150km (93 miles) from the DMZ, and are likely to be equipped with the Hwasong class of missiles, which can reach the U.S. mainland, according to the report.
"The United States and North Korea have yet to conclude an agreement that inhibits deployment of missiles by Pyongyang, never mind requiring their dismantlement".
But on Thursday, US ambassador to the United Nations Nikki Haley said North Korea postponed the meeting "because they weren't ready". They noted that Kim had not agreed to halt either nuclear weapons or missile development in negotiations with Trump or Secretary of State Mike Pompeo.
The bases are scattered across the country in underground facilities tunneled in narrow mountain valleys, according to the CSIS researchers, and designed for mobile missile launchers to quickly exit and move to previously prepared launch sites.
"Talk of "secrets" or "undeclared" or "deception" can possibly bring misunderstanding at this moment when there is a need for US North Korea dialogue, because it can block the talks and undermine the opening of negotiation", the Blue House official said.
Trump has often taken credit for North Korea halting tests of nuclear weapons and ballistic missiles. It's exactly what Kim said he would do.
Beyond Parallel classifies the operational North Korean missile bases into three zones, or "belts", with the closest, where Sakkanmol is located, considered to be "tactical". "The last round of tests under Kim Jong-un were not just coming from missile test sites".
"The intelligence authorities of South Korea and the USA have far more detailed information from military satellites and are closely monitoring (it)", Kim said.
"I continue to believe that Pyongyang does not intend to give up its nuclear weapons capability", said Revere.