AFP GETTY•DIGITAL GLOBE•38 NORTH Satellite images show increased activity at North Korea's nuclear site

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The chilling images reveal “unusually high levels of activity” at Kim Jong-un’s Punggye-ri Nuclear Test Site. Experts say the images could mean a nuclear test is “imminent” as US Secretary of State Rex Tillerson calls for North Korea to destroy its nuclear arsenal. North Korea has conducted five declared and remotely detected underground nuclear tests at its dedicated nuclear test site at Punggye-ri in mountainous terrain in the northeast of the country over the last decade.

Over the past four weeks, there have been unusually high levels of activity at the Punggye-ri Nuclear Test Site Joseph S Bermudez Jr and Jack Liu

The latest images suggest recent activity around the North Portal, which has been used for previous tests with unidentified equipment now at the site. Joseph S Bermudez Jr and Jack Liu from North Korea think tank 38 North which monitors the hermit kingdom, said: “Over the past four weeks, there have been unusually high levels of activity at the Punggye-ri Nuclear Test Site, especially around the North Portal—the tunnel in which the four most recent nuclear tests have taken place.

DIGITAL GLOBE•38 NORTH Activity continues at the North Portal

“New commercial satellite imagery from April 2 indicates continued activity in this area and at the Main Administrative Area, while the rest of the facility remains quiet. “This pattern of activity could mean a sixth nuclear test is imminent, but the imagery does not provide any definitive evidence of the installation of a nuclear device or the exact timing of such a test.”

DIGITAL GLOBE•38 NORTH Possible new dumping observed at the North Portal spoil pile

DIGITAL GLOBE•38 NORTH Probable personnel in formation or equipment in rows at the Main Administrative Area

Journalists visiting North Korea have have been told to prepare for a "big and important event" on Thursday as Pyongyang marks the 105th birth anniversary of its founding president Kim Il Sung on April 15, North Korea's biggest national day called "Day of the Sun”. North Korea often marks important anniversaries with military parades and tests of its nuclear or missile capabilities. The increased movement comes as tensions with the US threaten to spill over with a US Navy strike group heading towards the Korean peninsula.

Satellite images show Kim Jong-un escalating nuclear action Wed, April 12, 2017 New satellite images show in great detail how Kim Jong-un is escalating North Korea's nuclear weapons programme. Play slideshow DigitalGlobe/Getty Images 1 of 13 Figure 1. Activity continues at the North Portal