WASHINGTON, April 2 (Reuters) - Satellite images show a high level of activity at North Korea's main nuclear site, a U.S. think tank reported on Saturday after the North Korean leader ordered an increase in production of bomb fuel to expand the country's nuclear arsenal.

The Washington-based 38 North North Korea monitoring project said the activity it had spotted, based on images from March 3 and 17, could indicate that an Experimental Light Water Reactor (ELWR) at the Yongbyon site was nearing completion and transition to operational status.

The report said the images showed that a 5 megawatt reactor at Yongbyon continued to operate and that construction had started on a support building around the ELWR. Further, water discharges had been detected from that reactor's cooling system. New construction had also started around Yongbyon's uranium enrichment plant, likely to expand its capabilities.

"These developments seem to reflect Kim Jong Un's recent directive to increase the country's fissile material production to expand its nuclear weapons arsenal," the report added, referring to the North Korean leader.