Recent satellite imagery suggests North Korea has been constructing a new ballistic missile submarine and may be preparing for testing, a U.S. think tank said Wednesday.

The images confirm photographs and reports published by North Korean media outlets in late July, in which leader Kim Jong Un was shown inspecting a newly built submarine at what is believed to be Sinpo South Shipyard, according to the Center for Strategic and International Studies.

"The construction and commissioning of a true (ballistic missile submarine) capability would represent a significant advancement of the North Korean ballistic missile and nuclear threat and complicate defense planning in the region," the CSIS said.