SEOUL • Over the years, outside analysts have closely followed visits by North Korean leaders to factories, farms and military units to discern the regime's policy priorities.
The sleuthing is challenging: North Korean state news media often withholds the locations of these sites and their purposes, identifying them only by the names of their managers.
Now, two analysts based in the United States have located six such factories believed to be linked to North Korea's missile programme, visits to which by the country's leaders were deliberately obscured by state news media to thwart Washington's intelligence-gathering or cyber attacks.
The factories and their operations were discovered through a painstaking digital examination of open-source data.
"North Korea may be reluctant to share those locations precisely to make them harder to target," Mr Jeffrey Lewis, an expert on North Korea at the Middlebury Institute of International Studies in Monterey, California, said in a report published on Thursday.
"In other cases, however, the visits may have been related to the development of new missile-related systems that North Korea was not yet prepared to reveal."
The report about the sites comes as US Secretary of State Mike Pompeo prepares to meet North Korea's nuclear negotiator Kim Yong Chol to discuss steps towards denu-clearisation in the North that could lay the groundwork for a second meeting between US President Donald Trump and North Korean leader Kim Jong Un.
Mr Lewis worked with his colleague David Schmerler, often matching videos and photographs released by the North's state news media with commercial satellite imagery and details from visits by North Korean leaders to known factory sites.
Their report included map coordinates for the six plants, three of which turned out to be next to sites of important missile tests overseen by Mr Kim.
Since he took power in 2011, Mr Kim has rapidly accelerated his country's missile programme, which culminated in a series of test flights of its Hwasong-14 and Hwasong-15 long-range missiles in 2017, some of them believed to be capable of reaching North America.
At his meeting with Mr Trump last June in Singapore, Mr Kim vowed to work towards the complete denuclearisation of the Korean peninsula if Washington met his conditions, such as lifting sanctions and providing security guarantees.
But while he is engaged in diplomacy, Mr Kim has yet to announce a timetable for dismantling his nuclear arsenal. Instead, his country is still operating its missile bases and is suspected of continuing to improve its missile capabilities, despite Mr Trump's claims of progress in efforts to denuclearise the North.
The work by Mr Lewis and Mr Schmerler helps unveil the secretive nature of the North Korean missile programme.
US intelligence officials believed some of the plants produced armoured vehicles, light aircraft, machine tools or textiles. But until now, their probable links to the North's missile programme had not been publicised.
North Korea keeps many of its weapons-related facilities underground to protect them from outside monitoring or attacks in case of war. One factory the researchers located was believed to have two campuses, with the underground portion hidden near a textile plant.
"In some cases, Kim was visiting factories that are largely located underground and the effort was likely an effort to keep the location of the underground plant a secret," Mr Lewis said.