SEOUL (Reuters) - A U.S. think tank said on Monday it had identified at least 13 of an estimated 20 undeclared missile bases inside North Korea, underscoring the challenge for American negotiators hoping to persuade Pyongyang to give up its nuclear weapons and long-range missiles.

In reports released by the Washington, D.C.-based Center for Strategic and International Studies (CSIS), researchers said maintenance and minor infrastructure improvements had been observed at some of the sites despite the negotiations.

North Korean leader Kim Jong Un and U.S. President Donald Trump pledged to work toward denuclearization 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 U.S. and South Korean negotiators have yet to elicit from Pyongyang a concrete declaration of the size or scope of the weapons programs, 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.

A State Department official, asked whether those hidden sites went against the spirit of the summit and whether North Korea must give them up, said Trump had made clear that “should Chairman Kim follow through on his commitments - including complete denuclearization and the elimination of ballistic missile programs - a much brighter future lies ahead for North Korea and its people”.

Slideshow ( 10 images )

‘CLOSELY WATCHING’

Kim Eui-kyeom, a spokesman for the presidential Blue House in Seoul, said South Korean and U.S. intelligence officials had been “closely watching” the sites using military satellites and that the CSIS report contained “nothing new”.

He specifically criticized any suggestion that the bases constituted a “deception” by the North Koreans, or that there was any agreement that required Pyongyang to declare the existence of the bases.

“North Korea has never promised to shut down this missile base,” Kim Eui-kyeom said in a statement, citing one base described in detail by the CSIS researchers. “It has never signed any agreement, any negotiation that makes shutting down missile bases mandatory.”

An official with South Korea’s Joint Chiefs of Staff also told reporters that Seoul is “familiar” with the sites identified in the report but declined to confirm whether intelligence had indicated any recent changes at the bases.

North Korea called off a meeting with U.S. Secretary of State Mike Pompeo in New York last week. State media said on Monday the resumption of some small-scale military drills by South Korea and the United States violated a recent agreement aimed at lowering tensions on the Korean peninsula.

Slideshow ( 10 images )

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 denuclearization 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.

“North Korea’s decommissioning of the Sohae satellite launch facility, while gaining much media attention, obscures the military threat to U.S. forces and South Korea from this and other undeclared ballistic missile bases,” it said.