North Korea has built a secret military base near its border with China that may house Pyongyang's newest long-range ballistic missiles, according to new research.

The "undeclared" Sinpung-dong Missile Operating Base lies about 27 kilometers from the Chinese frontier, the Washington-based Center for Strategic and International Studies (CSIS) said in a report published Wednesday.

The facility in North Pyongan province likely houses six to nine nuclear-capable intercontinental ballistic missiles (ICBMs) and their launchers, the study said.