Motor tests to improve long-range missiles have been conducted in North Korea, a U.S. think tank said on Monday.

Experts at the U.S.-Korea Institute of John Hopkins University analyzed satellite photographs to determine that North Korea has continued weapon testing at a rocket launch site. 

According to the BBC, satellite imagery "indicates that North Korea continues to develop long-range missiles," involving "liquid-fueled, first stage engines". 

News of North Korea's military failure earlier this year was broadcast internationally. In April, the Unha-3, a 30m (or 100 ft) rocket, disintegrated and crashed into the sea shortly after launch from the Tongchang-ri base. 

The U.S., South Korea and Japan all criticized North Korea's decision to test the long-range missiles. In 2009, the UN Security Council, including permanent member and North Korean ally China, unanimously condemned launches, and agreed to tighten existing sanctions. As a result, North Korea abandoned nuclear disarmament talks in protest. Officials in Pyongyang claimed that the tests were merely an attempt to launch a satellite into orbit. 

The American think tank says that North Korea has carried out at least two more engine tests between April and September after examining commercial images of the Sohae launch station. Burnt vegetation, orange residue and movement of several fuel tanks indicate, according to the think tank, that rocket engines had been tested.

Both Seoul and Washington are watching North Korea's rocket technology development closely. Though the technology is still considered relatively primitive, experts believe that if development is successful, the rocket has the potential to reach not only the capital of South Korea, which is close to the North Korean border, but even the United States.

Nick Hansen, an expert on imagery analysis for the Institute, said that North Korea could be planning its tests tocommence after the South Korean presidential elections are finished in December.

"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," Hansen said on the Institute blog.