A U.S. research institute says an upgrade of North Korea's main rocket launch pad could be complete by April meaning it would be ready for new launches.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says new satellite imagery shows modifications underway at the west coast site of Sohae mean it could eventually accommodate rockets up to 50 meters long. The analysis was published Thursday on the institute's website, 38 North.

North Korea is not thought to have such a large rocket, but in December 2012 it launched into space the 30-meter-long Unha-3. The U.S. viewed that as a worrying sign of the North's development of ballistic missile technology.

The website reports no sign of launch preparations as of late January. Preparations can take about six weeks. (AP)