North Korea has been developing its long-range ballistic missile since Pyongyang’s failed rocket launch in April, a US think tank said Monday after reviewing recent satellite imagery indicating activity in Sohae satellite launch station.

The images taken between April and September showed that 34 fuel tanks had been moved and vegetation appeared to be burned, while orange residue was seen next to a flame trench, suggesting that Pyongyang had carried out at least two tests of large motors and worked on a launch platform. The “38 North” report by the US-Korea Institute at Johns Hopkins School of Advanced International Studies was released Monday.

Nick Hansen, who wrote the report, said the tests were likely the first-stage engines of the Unha-3 – the rocket used in the failed launch – or the new, bigger KN-08 long-range missiles.

South Korea’s defense minister Kim Kwan-Jin told AFP last week that the North had completed preparations for another nuclear test and long-range missile launches.

North Korean missiles are believed to have a rang of up to 4,160 miles and could reach some parts of Alaska, its Defense Ministry said. Pyongyang had threatened that its long-range missiles were capable of striking the US after Washington signed an accord with Seoul on acquiring longer-range missiles pointed to the North. However, Pyongyang’s inconsistent launch record means there are doubts.