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.