Construction of fuel storage bunkers at North Korea's rocket launch site appear to be near completion as a three-year project to expand the site draws to an end, a U.S. research institute said Wednesday.

Commercial satellite imagery taken of the Sohae Satellite Launching Station in the North's northwest from October through late November indicates that construction of new propellant (fuel/oxidizer) bunkers continues and is nearing completion, 38 North said in a report.

"With the upgrade program nearing completion, North Korea will be ready to conduct further activities at Sohae, including space launches, by the first quarter of 2016 should the leadership in Pyongyang decide to do so," 38 North said in a report.

South Korean military and intelligence officials have said they believe the upgraded facility can be used for the launch of long-range missiles twice the size of the 30-meter-long Unha-3, which put a satellite into orbit in December 2012.

Pyongyang is believed to have developed advanced ballistic missile technologies through a series of test launches, including the 2012 launch that is considered the most successful so far. That test sparked fears that the North has moved closer to ultimately developing nuclear-tipped missiles that could potentially reach the mainland U.S. (Yonhap)