The Institute for Science and International Security (ISIS), a U.S. nuclear disarmament think tank, said that it detected new construction activity at North Korea’s Yongbyon nuclear site in addition to nuclear reactor-related buildings.

The institute said upon analysis of satellite images that these construction and renovation activities took place after the April 2009 departure of IAEA monitors and U.S. experts.

The institute said a look at the satellite pictures taken in March shows that two buildings were built southeast of the uranium enrichment plant, and foundation work for two new buildings could be seen.

The institute said the construction work could be linked to uranium enrichment, uranium conversion, and processing fuel rods, making the case for North Korea to provide information on the new buildings.