SEOUL, South Korea — Satellite imagery of a North Korean nuclear research site suggests improvements have progressed at a “rapid pace” in recent months, a report from a monitoring group said Wednesday, but the level of current operations at the facility remains unclear. The analysis by the group 38 North, which closely follows North Korea affairs, offered a snapshot of the Yongbyon Nuclear Scientific Research Center on June 21 – nine days after North Korean leader Kim Jong Un signed a declaration to “denuclearize” after the Singapore summit with President Donald Trump. The 38 North report said the satellite imagery leaves doubts about the Yongbyon reactor’s “operational status” because of reduced discharge of cooling water, steam emissions and other factors.