A US think tank says it has identified at least 13 of an estimated 20 undeclared missile operating bases inside North Korea, underscoring the challenge for American negotiators hoping to persuade Pyongyang to give up its nuclear weapons and long-range missiles.
Researchers from the Washington-based Centre for Strategic and International Studies (CSIS) said maintenance and minor infrastructure improvements had been observed at some of the sites despite the negotiations.
North Korean leader Kim Jong-un and US President Donald Trump pledged to work towards denuclearisation at their landmark June summit in Singapore but the agreement was short on specifics and negotiations have made little headway.
Trump said on Twitter shortly after that summit "there is no longer a Nuclear Threat from North Korea".
North Korea declared its nuclear force "complete" and halted missile and nuclear bomb testing earlier this year but US and South Korean negotiators have yet to elicit from Pyongyang a concrete declaration of the size or scope of the weapons programmes, or a promise to stop deploying its existing arsenal.
North Korea has said it closed its Punggye-ri nuclear testing site and the Sohae missile engine test facility. It also raised the possibility of shutting more sites and allowing international inspections if Washington took "corresponding measures", of which there has so far been no sign.
An official with South Korea's Joint Chiefs of Staff told reporters that Seoul is "familiar" with the sites identified in the report but declined to confirm whether intelligence has indicated any recent changes at the bases.
The sites identified in the CSIS report are scattered in remote, mountainous areas across North Korea and could be used to house ballistic missiles of various ranges, the largest of which is believed to be capable of striking anywhere in the United States.
"Missile operating bases are not launch facilities," the report said.
"While missiles could be launched from within them in an emergency, Korean People's Army operational procedures call for missile launchers to disperse from the bases to pre-surveyed or semi-prepared launch sites for operations."
None of the missile bases has been acknowledged by North Korea and analysts say an accurate disclosure of nuclear weapons and missile capabilities would be an important part of any denuclearisation deal.
Sakkanmol, the site closest to the border with South Korea and its capital, Seoul, appears to be "active and being reasonably well maintained", the report found.
