Satellite imagery indicates North Korea has been testing rocket engines, a sign it continues to develop long-range missiles, a US academic institute says.
The analysis provided on Monday is based on satellite images taken as recently as late September of the Sohae site on the northwest coast. In April, North Korea launched a rocket from there in a failed attempt to propel a satellite into space in defiance of a UN ban.
The analysis on the website of the US-Korea Institute at Johns Hopkins School of Advanced International Studies said it remains unclear whether North Korea was preparing another rocket launch but predicted it may embark on new rocket and nuclear tests in the first half of 2013.
The analysis underscores the challenges posed by North Korea's weapons programmes to the United States and its allies as President Barack Obama heads into his second term.
Washington's most recent attempt to negotiate a freeze in North Korea's nuclear programme and a test moratorium in exchange for food aid collapsed with the April launch, which the US regarded as a cover for testing ballistic missile technology.
The US-Korea Institute analysis concludes that since the failed launch of the Unha-3 rocket, which disintegrated shortly after take-off, North Korea has conducted at least two, and possibly more, tests of large rocket motors.
"Pyongyang's large motor tests are another clear sign that its missile programme 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 the 38 North website.
An April 9 satellite image shows what appear to be dozens of fuel tanks near a stand used for conducting tests of rocket engines. A September 17 image shows the tanks are no longer there, and a flame trench has been 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.
The analysis was written by Nick Hansen, a retired expert in imagery technology with experience in national intelligence.
He concludes the tests were likely of the first-stage engines of the Unha-3 or the bigger KN-08 missile first viewed in a spring parade in Pyongyang.
The images also show construction work on a launch tower at Sohae to enable it to accommodate even larger rockets than the Unha-3 or KN-08.