SEOUL (NYTIMES) - 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 withhold 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 (Jan 18).

"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, Mr Kim Yong Chol, to discuss steps toward denuclearisation in the North that could lay the groundwork for a second meeting between 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 Korean 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.

Mr Kim did so while frequently visiting weapons-related facilities, inspecting tests and feting officials and engineers involved in weapons development with parades, gala parties and heroes' titles.

After its last test of the Hwasong-15 intercontinental ballistic missile, conducted in November 2017, Mr Kim announced a moratorium on nuclear and long-range missile tests, saying North Korea no longer needed them because it had completed its nuclear deterrent.

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 for North Korea. In a New Year's Day speech this year, Mr Kim indicated that his country was no longer making nuclear weapons.

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 armored vehicles, light aircraft, machine tools or textiles. But until now, their probable links to the North's missile programme had not been publicised.

From 2012 to 2016, Mr Kim made five publicised visits to what North Korean state news media identified as a machine plant "managed by Ho Chol Yong" in north-west North Korea that was undergoing a significant expansion, Mr Lewis and Mr Schmerler said.

According to their research, North Korea actually used this location to launch its Pukguksong-2 missile in February 2017. The test marked a major leap forward for the North because the medium-range ballistic missile used solid fuel, which makes it easier to hide, transport and launch, and harder for the US to target in a pre-emptive strike.

The test was dramatic enough at the time that aides to Mr Trump and Prime Minister Shinzo Abe of Japan interrupted their dinner at the Mar-a-Lago resort in Florida to bring them early reports of the launch.

North Korea also used the location to flight-test its Hwasong-12 intermediate-range ballistic missile in May 2017, another major stride for the North's missile programme. The plant is now believed to be involved in the development of tracked launch vehicles for missiles, Mr Lewis and Mr Schmerler said.

Mr Kim visited another machine plant, supposedly "managed by Jon Tong Ryol", in Panghyon in north-west North Korea in 2014 and 2015. When he visited the area again in July 2017, the North Korean leader watched the launch of the Hwasong-14, the country's first major test of an intercontinental ballistic missile, which Mr Kim at the time mockingly called his Fourth of July "gift" for Mr Trump.

In their report, Mr Lewis and Mr Schmerler also located a machine plant said to be managed by Ri Chol Ho in Pyongyang, the North Korean capital, that makes integrated circuits.

Mr Kim visited the plant three times from 2013 to 2016, and North Korean state news media has described it as "a nice plant in a park".

North Korea may well have chosen to obscure the location because of fears that a factory producing integrated circuits, crucial for space and missile applications, would be a target for US cyber attacks, the researchers said.

The Pentagon did not immediately respond to a request for comment on the report's findings.

North Korea keeps many of its weapons-related facilities underground to protect them from outside monitoring or attacks in case of war.

One factory Mr Lewis and Mr Schmerler located was believed to have two campuses, with the underground portion hidden near a textile plant. They learnt that North Korean state news media used different names when Mr Kim visited the aboveground and underground facilities there, Mr Lewis said by e-mail.

"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," he said.