Today, a new report from a Washington think-tank identified more than a dozen hidden bases in North Korea that could be used to disperse mobile launchers for ballistic missiles in the event of a conflict.
Are these bases evidence that North Korea is cheating on the agreement it reached in June, when US President Donald Trump met North Korean leader Kim Jong Un in Singapore? Analysts say the answer is no - although there are plenty of caveats.
"Kim hasn't broken any promises," said Jeffrey Lewis, a nonproliferation expert at the Middlebury Institute of International Studies at Monterrey. "Instead, he's making good on one of them - to mass produce nuclear weapons."
Joseph Bermudez, Victor Cha and Lisa Collins of the Centre for Strategic and International Studies used satellite imagery and interviews with North Korean defectors and government officials to identify 13 missile bases. They say there are seven more bases that remain hidden.
The bases, which are located in "mountainous terrain, often spread out within narrow dead-end valleys," they write, could be used to deploy mobile missile launchers, which would be extremely difficult for other nations to track and stop before the missiles could be fired.
The analysts looked specifically at the Sakkanmol Missile Operating Base, located only 135km from Seoul. "As of November 2018, the base is active and being reasonably well-maintained by North Korean standards," the report noted.
North Korea dramatically improved its missile technology with provocative test launches last year. Western experts eventually concluded that Pyongyang now has the ability to hit most of the United States, although it remains unclear if it could produce a nuclear weapon small enough to fit atop a missile.
After a tense 2017, diplomacy produced a thaw at the start of this year.
North Korea announced it would put a halt to its weapons testing and dismantled a nuclear test site and a satellite launch facility thought to play a role in the country's ballistic missile programme.
When Kim met with Trump in Singapore, the two leaders agreed to build a "lasting and stable peace regime on the Korean Peninsula" and to "work toward complete denuclearisation" of the peninsula.
The day after the summit, Trump wrote on Twitter that there was "no longer a nuclear threat from North Korea." In public, he has largely kept this positive view of negotiations. "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 report is the latest evidence that while North Korea has indeed stopped its missile testing, it is far from dismantling its weapons facilities.
Indeed, it appears to be adding to its stockpile: US intelligence reports from the northern summer found that North Korea had begun producing new missiles at a factory, and Secretary of State Mike Pompeo acknowledged during Senate testimony that Pyongyang "continues to produce fissile material".
Duyeon Kim, an adjunct senior fellow at the Centre for a New American Security, said while these developments violate UN Security Council resolutions, they do not breach Kim's agreement with Trump. "North Korea didn't break any promises with Trump because there's no nuclear deal in place yet with Washington," she said.
"I realise I am a broken record, but North Korea has never offered to abandon its nuclear weapons," said Lewis.
"What North Korea has offered is the beginning of a process that might - might - someday lead to an outcome like that."
The State Department did not say whether they viewed the base as a violation of any agreements with the United States.
"President Trump has 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," a spokesman said in a statement.
There have been several recent signs that diplomatic efforts between the United States and North Korea were stalling. A meeting scheduled for last week between Pompeo and Pyongyang's nuclear negotiator, Kim Yong Chol, was postponed unexpectedly, only days after North Korean state media released a commentary that suggested missile testing could resume if progress was not made on talks.
Analysts have long believed that North Korea had undeclared or hidden facilities in their missile programme, which has made issues related to verification and inspections a major part of working level talks between the US and North Korea.
Pyongyang may have wanted the missile sites to be seen, suggested David Maxwell, a senior fellow at the Foundation for Defence of Democracies, to keep the pressure on in negotiations.
Kim Sung Han, the dean of Korea University's Graduate School of International Studies and a former South Korean vice-foreign minister, said that the CSIS report showed the need for a "verifiable declaration" of North Korea's nuclear and missile capabilities as the next step in negotiations.
However, he added, the report could also make the current stalemate between Washington and Pyongyang last "longer than expected".