North Korea is building fortified structure with what appears to be two covered docks that could shelter ballistic missile submarines, IHS Jane's Defence Weekly reports.

Located south of the Singpo shipyard and near the Mayang-do naval base on the east coast, the new facility could be the largest active military building project in North Korea presently, Jane's reported Friday.

Commercial satellite imagery reveals that construction of the base began sometime between August 2009 and November 2012, Jane's said.

Much of the 6,000-square-meter harbor seen in 2009 had been blocked off by a sea wall and filled in by November 2012, with soil from surrounding hills likely used for filling material, the report states.

Construction of the new pier had commenced by October 2013, Janes says, and the docks were taking shape by July 2014.

The steel structures over the docks were being set and concrete slabs had been laid to form the roofs by mid-2015.

May 2015 imagery showed the two pens are about 490 feet long, 32 feet wide and about 46 feet apart.

Satellite imagery from May 8 showed that pen construction had progressed to the point where portions were being covered with earth, and the new pier was nearing completion, Jane's reports.