The U.S.-based North Korea monitoring Web site 38 North said Friday that Pyongyang is likely preparing for its sixth nuclear test with significantly higher yields.

It said that commercial satellite imagery of the Punggye-ri nuclear test site shows that substantial tunnel excavation is continuing at the "North" Portal, which was previously the “West” Portal. The website said that portal provided support for the last four of the five declared underground nuclear tests the North conducted.

The report said the North Portal tunnels provide direct access under Mantap Mountain where overlying rock is available for test containment. It said that continued tunneling under the mountain has the potential for allowing North Korea to support additional nuclear tests of significantly higher explosive yields, perhaps up to 282 kilotons.

38 North forecasts the sixth nuclear test will be at least 14 times more explosive than the fifth test which produced an estimated yield of 15- to 12-thousand tons.

The North's very first nuclear test yielded 800 tons of explosive strength which continued to increase in following tests. 

Meanwhile, John Schilling, an engineering specialist at the U.S. Aerospace Corporation said in an interview with Radio Free Asia on Saturday that he believes North Korea will successfully test launch an intercontinental ballistic missile in two years and be able to deploy it by 2020. 

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