A U.S. think tank says it has identified at least 13 out of an estimated 20 undeclared missile operating bases inside North Korea.

The Center for Strategic and International Studies(CSIS) said in a report on Monday that maintenance and minor infrastructure improvements have been observed at some of the sites.

The sites are reportedly 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.

CSIS researcher Joseph Bermudez said in the report that Sakkanmol, the site closest to the border with South Korea and its capital Seoul, appears to be “active and being reasonably well maintained.”

None of the missile bases have been acknowledged by North Korea.