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Extensive excavations are taking place at Kim Jong-un’s Punggye-ri nuclear site, with satellite images revealing tunnels are being built under Mount Mantap at the site, which previously provided support for underground nuclear tests. Experts warn the new tunnels could support even more explosive tests. Frank Pabian and David Coblentz from North Korea think tank 38 North which monitors the hermit kingdom said: “The continued tunnelling under Mount Mantap via the North Portal has the potential for allowing North Korea to support additional underground nuclear tests of significantly higher explosive yields, perhaps up to 282 kilotons.”

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

The North Portal has the potential for allowing North Korea to support additional underground nuclear tests of significantly higher explosive yields, perhaps up to 282 kilotons Frank Pabian and David Coblentz

The think tank, a programme of the US-Korea Institute in Washington DC, warns the tests have become more powerful since October 2009. A security fence around the perimeter of the North Portal suggests it is “the primary test portal” with tunnels being dug at three other sites at Punggye-ri. 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, 38 North said.

REUTERS/ DIGITAL GLOBE Extensive excavations are taking place at Kim Jong-un’s Punggye-ri nuclear site

Analysts say new commercial satellite imagery suggest Pungyye-ri “is capable of handling a sixth nuclear test on short notice once a nuclear device and the associated monitoring equipment are emplaced”. Previously seen supplies and equipment have been moved with a large shipping container in its place at the North Portal with an “increase in a activity” at the command centre area. But despite the flurry of activity it is unclear when the next test will take place as tensions escalate between the US and Pyongyang over the growing nuclear and missile threat.

GOOGLE EARTH Event locations with tunnel portals and conjectured tunnel projections at Punggye-ri

Kim Jong-un has warned North Korea can test launch an intercontinental ballistic missile at any time from any location and threatened to unleash a “merciless” attack on the US if joint military exercises with South Korea continue. Once fully developed, a North Korean ICBM could threaten the continental United States, which is around 5,500 miles from the North. US and South Korean military chiefs have discussed operations if North Korea attacks large-scale joint drills between the allies.

REUTERS Kim Jong-un has warned North Korea can test launch an intercontinental ballistic missile at any time

Since last year Pyongyang has carried out two nuclear tests and a string of missile launches, including four in the last week. US secretary of state Rex Tillerson said tough measures were needed to tackle North Korea, as 20 years of “failed” policies had done little more than see the US $1.35bn worse off due to monster foreign aid payments. Speaking on Thursday in Japan, Mr Tillerson said: "So we have 20 years of failed approach.

Secretive images revealed as North Korea prepares for war Wed, April 5, 2017 A look at North Korea's firepower as it pursues its nuclear and ballistic missile programs in defiance of the United States and its allies. Play slideshow AFP/Getty Images 1 of 30 People watch a television news showing file footage of a North Korean missile launch, at a railway station in Seoul