A US think tank said on Monday it had 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, Reuters reported.

In reports released by the Washington, D.C.-based Center for Strategic and International Studies, researcher Joseph Bermudez said maintenance and minor infrastructure improvements have been observed at some of the sites, despite the ongoing negotiations.

“Missile operating bases are not launch facilities,” Bermudez wrote. “While missiles could be launched from within them in an emergency, Korean People’s Army (KPA) 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 have been acknowledged by North Korea, and analysts say an accurate disclosure of nuclear weapons and missile capabilities would be an important part of any denuclearization deal.