Researchers working on a project sponsored by the Center for Strategic and International Studies(CSIS) have published more details on what they claim are secret ballistic missile bases in North Korea. 

In the "Beyond Parallel" report published Monday, the researchers estimate North Korea has 20 undisclosed sites where it continues to develop its ballistic missile program. 

The report focused on the Sino-ri Missile Operating Base situated 212 kilometers north of the Demilitarized Zone, which it says houses the headquarters for the Korean People's Army Strategic Rocket Forces missile brigade.

However, it is not newly discovered as it has already been mentioned by South Korean media. 

Last November, the U.S. think tank said it identified at least 13 out of an estimated 20 undeclared missile operating bases inside North Korea, introducing extensive details on the Sakkanmol base, located closest to the South Korean border of all 20 bases. 

According to Yonhap News, a Joint Chiefs of Staff official said Tuesday that the Sino-ri base is among the sites being monitored by South Korea and the U.S. 

The CSIS report comes after an announcement that the U.S. and North Korea will hold a second summit next month.