A U.S. research institute says North Korea seems to have resumed efforts to build a light-water reactor within its nuclear complex in Yongbyon.

The Johns Hopkins School of Advanced International Studies made the assessment on Wednesday based on satellite photos of the nuclear complex taken on April 30th.

The Washington-based institute said construction of a light-water reactor proceeded at a significantly quick pace last year before it was suspended for three months from December. The institute added that construction seems to have resumed from the end of March.

However, the institute noted that the light water reactor wouldn't likely be up and running before 2014 or 2015. It said the construction efforts highlight that North Korea is focused on expanding its nuclear program despite international criticism.