A United States think tank on Monday said it had identified at least 13 of an estimated 20 undeclared missile operating bases inside North Korea.

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

The sites identified in the CSIS report were said to be scattered in remote, mountainous areas across North Korea and could be used to house ballistic missiles of various ranges, with the largest believed to be capable of striking anywhere in the United States.

North Korean leader, Kim Jong Un and US President, Donald Trump had during their landmark June summit in Singapore pledged to work toward denuclearization but the agreement, which was short on specifics and negotiations have made little headway.

Just last week, North Korea called off a meeting with US Secretary of State, Mike Pompeo in New York over the resumption of some small-scale military drills by South Korea and the United States which Pyongyang claimed violated a recent agreement aimed at lowering tensions on the Korean peninsula.