Increased activity at North Korea’s main nuclear test site during the past few days could mean the country is in final preparations for a sixth nuclear bomb test, according to a report from the U.S.-Korea Institute.

38 North, a website published by the U.S.-based think tank, released photos of the Punggye-ri nuclear test site taken March 28 that show vehicles moving around the area and water being pumped from a tunnel, likely to keep it dry for communications and monitoring equipment.

In a statement released with the photos, the group said the activity is “consistent with previous reports,” though it noted a large, new contingent of people “standing in formation” in the base’s administrative area.

The statement said, “Such a gathering has not been seen since January 4, 2013, which was followed by a nuclear test on February 12.”

According to 38 North, the North Koreans generally try to avoid activity when they know satellites are passing overhead, so for the satellites to capture images of them in formation “suggests that Pyongyang is sending a political message.”

During the weekend, the website reported the possibility of vehicles and trailers at the site being used to install a nuclear device for an underground test.

Despite United Nations sanctions forbidding North Korea from launching nuclear tests, the country has conducted five such tests and a series of missile launches. Last year, North Korea said it had gained the capability to mount a warhead on a ballistic missile.

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