SEOUL, South Korea — Recent commercial satellite imagery indicates that North Korea has completed a yearlong project to upgrade its main satellite launching station, which is widely believed to be a test site for its intercontinental ballistic missile program, a United States research institute said this week.

Construction has been underway at the Sohae Satellite Launching Station in Tongchang-ri in northwestern North Korea since late last year. It includes modifying the gantry tower and launchpad there, which analysts said would give North Korea a facility to launch a longer-range rocket that can carry a heavier payload.

North Korea successfully launched its Unha-3 space launch vehicle from the Sohae facility in December 2012, putting a small satellite into orbit. The launch increased fears that the country was inching toward acquiring the ability to build an intercontinental ballistic missile with a nuclear warhead.

“North Korea is now ready to move forward with another rocket launch,” the U.S.-Korea Institute at Johns Hopkins University said in a report published on its website on Wednesday. “Should a decision be made soon to do so in Pyongyang — and we have no evidence that one has — a rocket could be launched by the end of 2014.”