North Korea has conducted motor tests to improve its long-range missiles after a failed launch in April, a U.S. think tank said Monday after reviewing new satellite images.

Since the embarrassing flop in April, the communist regime appears to have carried out at least two tests of large motors needed for rockets and worked on a launch platform, the U.S.-Korea Institute at Johns Hopkins University said.

The institute examined commercial images of the Sohae satellite launch station between April and September and found that 34 fuel tanks had been moved and vegetation appeared to be burned, next to a flame trench stained with an orange residue.

Such fuel tests would boost development of engines for the Unha-3, the rocket which North Korea unsuccessfully launched in April, or what seemed to be a new, longer-range missile displayed at a military parade the same month.

Some analysts believe that a North Korean rocket, if successfully developed, could eventually reach the range to hit the United States.

Nick Hansen, an expert on imagery analysis, said that North Korea may step up action after elections in both the United States and South Korea, the regime's two primary foes.

"In the aftermath of the U.S. and South Korean presidential elections, Pyongyang may embark on a new round of activities in the first half of 2013, including rocket and nuclear tests that will contribute to further development of its nuclear deterrent," he wrote on the institute's blog, 38 North.

South Korea's defense minister, Kim Kwan-Jin, said last week that North Korea had completed preparations for another nuclear test and long-range missile launches.

However, 38 North in September reported a work stoppage at a new launch pad for intercontinental missiles -- possibly due to rain -- that could set the project back by up to two years.

North Korea defiantly went ahead with the rocket launch in April, saying it was trying to put a satellite in orbit, but it disintegrated just two to three minutes after blast-off.

The test put a halt to the latest international effort to engage the isolated state, with the United States calling off plans to deliver badly needed food assistance. (AFP)