North Korea is constructing two covered docks that could shelter ballistic missile submarines near the port city of Sinpo on its east coast, IHS Jane's reported Friday.

Located about 2 kilometers south of Sinpo, close to the Mayang-do Naval Base, the new structure may be the largest active military building project in North Korea at the moment, the defense research firm said.

Commercial satellite imagery from May 8 shows that construction, which began sometime between August 2009 and November 2012, was ongoing on the fronts of both pens, it said, and a barge was tied to the seawall.

North Korea's 2,000- to 2,500-ton Sinpo-class submarines are believed to have been created through reverse engineering of Soviet-made diesel-powered subs.

The isolated country fired what appeared to be a ballistic missile launched from a submarine off its east coast on July 16, but the projectile failed in flight, South Korea's military said, amid renewed tensions in the region.