North Korean leader Kim Jong-un observes what the regime claims is a hydrogen bomb during an inspection at its Nuclear Weapon Institute in this captured image from the North's Korea Central TV broadcast on Sept. 3, 2017. Yonhap

Recent commercial satellite imagery indicates North Korea continues to expand and refurbish its Yongbyon nuclear complex, reinforcing concerns the country is acting on leader Kim Jong-un's recent call for an "exponential" increase in its nuclear arsenal, 38 North, a U.S. think tank, said Friday.

The report based on satellite imagery taken April 20 said significant construction and improvement activities have been detected throughout the Yongbyon Nuclear Scientific Research Center, particularly around the Experimental Light Water Reactor (ELWR), the 5 MWe Reactor and within the uranium conversion area.

The report written by Peter Makowsky, Jack Liu and Olli Heinonen also noted improvements to the Kuryong River causeway channel dam and other new building projects across the complex, indicating significant investment in improving North Korea's fissile material production capabilities.

The researchers also observed water discharge continues south of the ELWR pump house, suggesting that testing or cooling operations are still in progress.

In late March, North Korean leader Kim Jong-un ordered the expansion of the production of weapons-grade nuclear materials for an exponential increase in its arsenal.

He called for "flawless" preparations for the use of nuclear weapons "anytime and anywhere," as he received a briefing from the Nuclear Weapons Institute, according to the official Korean Central News Agency. (Yonhap)