South Korea's National Intelligence Service said movements of supply transport vehicles were detected at North Korea's missile research facilities at Sanumdong and that it practically viewed it as missile-related activity, a South Korean newspaper reported on Thursday.
The National Intelligence Service also confirmed during a briefing to lawmakers on Tuesday that North Korea continued to enrich uranium before last month's summit between U.S. President Donald Trump and North Korean leader Kim Jong Un, said newspaper JoongAng Ilbo, citing a lawmaker.
It was the first official confirmation by South Korean authorities that North Korea has been continuing to enrich uranium, JoongAng said.
Sanumdong is known as the production site of North Korea's intercontinental ballistic missile Hwasong-15, seen as the country's first intercontinental ballistic missiles capable of reaching the United States.
The news comes as two U.S. think tanks and South Korea's Yonhap News Agency reported on Tuesday that work was underway to restore part of North Korea's Sohae Satellite Launching Station even as Trump met Kim held a second summit in Hanoi last week.
Trump said on Wednesday he would be very disappointed in Kim if reports about rebuilding at a rocket launch site in North Korea were true.