US analysts said Monday they have located more than half of an estimated 20 secret North Korean missile development sites, highlighting the challenge the Trump administration faces in ensuring that North Korea complies with any eventual agreement to end its nuclear and missile programs.

The findings, which were released amid signs the administration’s negotiations with the North have stalled, draw on commercial satellite imagery and identify 13 secret facilities used to produce missiles and related technology.

They suggest that Pyongyang is continuing its missile work, even though it has halted test launches in what US President Donald Trump and his administration have claimed as a success since his historic June meeting with North Korean leader Kim Jong-un in Singapore.

Although the sites are not launch facilities and in some cases are rudimentary, the authors of the report from the Center for Strategic and International Studies say they are hidden and illustrate the scope of the North’s weapons program and the country’s determination to conceal its military might.

“The dispersed deployment of these bases and distinctive tactics employed by ballistic missile units are combined with decades of extensive camouflage, concealment and deception practices to maximise the survival of its missile units from pre-emptive strikes and during wartime operations,” they said.

The authors say that the sites, which can be used for all classes of ballistic missile, therefore should be declared by North Korea and inspected in any credible, verifiable deal that addresses Pyongyang’s most significant threats to the United States and its allies.

North Korea analysts not involved in the report said the findings were not surprising given Pyongyang’s past activities but were still cause for concern.

They noted that Kim had not agreed to halt either nuclear weapons or missile development in negotiations with Mr Trump or Secretary of State Mike Pompeo.

“The fact that North Korea has continued to build nuclear weapons and ballistic missiles in the midst of high-level diplomacy with China, South Korea, and the US should not come as a surprise,” said Abraham Denmark, the Asia program director at The Wilson Center.

“Despite all the summitry, North Korea is just as dangerous today as it was a year ago.” “Improving relations with Pyongyang may be a laudable goal, but any claim that the North Korean nuclear and missile threats have been solved is either wishful thinking or purposefully deceptive,” he said.

“Interesting but unsurprising report,” said Kelsey Davenport, director for nonproliferation policy at the Arms Control Association.

“Kim Jong-un only committed voluntarily to halt long-range missile tests,” said Mr Davenport.

The report was released less than a week after North Korea abruptly called off a new round of negotiations with Mr Pompeo that had been set for Thursday in New York.

The cancellation, which the US ascribed to scheduling issues, followed threats from North Korean officials to resume nuclear and missile testing unless US sanctions are lifted.

The administration has said repeatedly that sanctions will not be lifted until a denuclearisation agreement is fully implemented.

“It’s not like these bases have been frozen,” Victor Cha, who leads CSIS’s North Korea program, told The New York Times, which first reported on the study.

“Work is continuing. What everybody is worried about is that Trump is going to accept a bad deal — they give us a single test site and dismantle a few other things, and in return they get a peace agreement.”

Mr Cha had been in line for appointment as US ambassador, but was dropped because of disagreement with the Trump administration’s approach.

While US sanctions on North Korea remain in place, enforcement by traditional trading partners China and Russia has relaxed since the summit, US officials have acknowledged.

The bases, which are scattered across the country, are located in underground facilities tunnelled in narrow mountain valleys, according to the researchers.

They are designed so that mobile missile launchers can quickly exit the underground facilities and move to previously prepared launch sites.

The bases are arranged in three belts across North Korea, according to the report, with those for strategic missiles deep inside the country.

Medium-range missiles capable of striking Japan and all of South Korea reportedly are deployed in an operational belt 90 to 150 kilometres north of the demilitarised zone.

Shorter range missiles fit into a tactical belt 48km to 88km from the DMZ.

The report included a detailed profile of one such tactical missile operating base, illustrated with commercial satellite imagery, that is just 135 miles northwest of Seoul.

Led by Joseph Bermudez, an authority of North Korea, the researchers’ findings were based on satellite imagery, defector interviews and interviews with intelligence and government officials.

Mr Trump has said he hopes to meet again soon with Kim, but there are signs of friction in the negotiations with North Korean officials