Fresh satellite images show North Korea has completed upgrading its main satellite site to handle far larger rockets, suggesting a possible launch by the end of the year, a US think tank said Thursday.

"North Korea is now ready to move forward with another rocket launch," the US-Korea Institute at Johns Hopkins University said on its closely followed 38 North website.

While stressing there was no evidence of a planned launch, the institute said Pyongyang now had the capability to send up a rocket by the end of 2014.

Satellite analysis has shown a major construction programme underway at North Korea's Sohae Satellite Launching Station since mid-2013, focused on upgrading facilities to handle larger, longer-range rockets with heavier payloads.

North Korea successfully put a satellite into orbit with its Unha-3 carrier in December 2012.

That launch was condemned by the international community as a disguised ballistic missile test and resulted in a tightening of UN sanctions.

The US-Korea Institute believes the completed upgrade would allow the Sohae site to handle rockets of up to 50 metres (165 feet) in length -- almost 70 percent longer than the Unha-3.

But such a rocket is still believed to be several years from becoming operational, meaning that a repeat Unha-3 launch would be more likely in the short-term, the institute said.

The latest images also suggested further engine-tests in August for an under-development intercontinental ballistic missile (ICBM).

"It remains unclear how successful these tests have been," the institute said. "However, rocket motor tests are typically conducted prior to full-scale test launches."

Development of a working ICBM would be a game-changing step, bringing the continental United States into range and adding a whole new threat level to the North's regular nuclear-strike warnings.