Story highlights Satellite imagery could show renewed activity at the Yongbyon facility in North Korea

Secretary Mattis will be visiting South Korea and Japan this week

It's his first visit abroad as secretary of defense

Hong Kong (CNN) North Korea appears to be restarting a plutonium reactor just as US Secretary of Defense James Mattis prepares to visit neighboring South Korea and Japan.

An analysis of new satellite imagery from 38 North, a North Korea tracking project associated with the School of Advanced International Studies at Johns Hopkins University, concludes that country is preparing to restart the reactor at the Yongbyon nuclear facility.

The images show that most of a river near the reactor is frozen over, except for where water originating from a reactor outlet mixes with the river -- indicating that the reactor is likely operational.

Source: DigitalScope/38 North/Getty Images

Mattis' visit to South Korea and Japan will be his first overseas trip as secretary of defense, making it a symbolic show of support for two of the US' most important allies in the region who have concerns about how President Donald Trump will approach relations in the Pacific.

Trump has said that he wants to rethink the US role in the region. He's publicly mused whether it's worth continuing to support Japan and South Korea without reevaluating what the US gets in return.

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