Last month, US defence Secretary Leon Panetta said North Korea continues to prepare for such tests, and the North, angered by Washington's recent agreement to let Seoul possess missiles capable of hitting all of its territory, has recently claimed that the US mainland is within range of its missiles.
According to South Korea's defence Ministry, North Korean missiles are believed to have a range of up to about 6,700 kilometres, putting parts of Alaska within reach. But the North is not believed to have mastered the technology needed to hit a distant target and miniaturize a nuclear warhead to mount on a missile. The North has a spotty record in test launches, raising doubts about whether it is truly capable of a long-distance attack.
The 38 North analysis concludes that since the failed launch on April 13 of the Unha-3 rocket that disintegrated shortly after takeoff, the North has conducted at least two, and possibly more, tests of large rocket motors at a test site about 1 kilometre away. The tests are critical for the development of new rockets.
"Pyongyang's large motor tests are another clear sign that its missile program is moving forward. Whether there will be another long-range missile test this spring remains unclear but is a distinct possibility," said Joel Wit, a former US State Department official and editor of 38 North.
An April 9 satellite image shows what appear to be dozens of fuel tanks positioned on each side of a fuel propellant storage building next to a stand used for conducting tests of rockets. The tanks were still there April 13, indicating they were not intended for the nearby rocket launch.
An image of the same site on Sept. 17 shows the tanks are no longer there, and a flame trench is stained orange and surrounding vegetation has been burned from the exhaust of an engine.
An image from September 28 indicates a further test has taken place. There's been more discoloration of the flame trench and burning of vegetation, and a small crane is at the site that could have been used to put the rocket engine into position. Just off a road leading from the testing area, a large truck and trailer is parked and is carrying a white 3.2-metre-long object that 38North says could be an engine that has recently been tested.
The analysis was written by Nick Hansen, a retired expert in imagery technology with a 43-year experience in national intelligence.
He concludes the tests were likely of the first-stage engines of the Unha-3 or the new, bigger KN-08 long-range missile first viewed in a military parade in Pyongyang shortly after the April launch attempt.
The capabilities of the KN-08 and whether it could pose a potential threat to the continental United States remains unclear. Some analysts have also questioned whether the half-dozen of KN-08 missiles shown at that parade were genuine or just rigged up for show.
38North says the September 28 images also show construction work on the upper platform of a launch tower at Sohae to enable it to accommodate even larger rockets than the Unha-3 or KN-08.
-AP