WASHINGTON (Reuters) - North Korea appears close to completing an upgrade of its rocket launch site at Sohae, which could be ready to launch larger rockets from the beginning of 2016, a U.S. research institute said on Wednesday, citing recent satellite images.

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North Korea has tested three nuclear devices in defiance of international sanctions and is thought to be trying to develop an intercontinental ballistic missile mounted with an atomic warhead that could hit targets in the mainland United States.

A report on 38 North, a website run by the U.S.-Korea Institute at Johns Hopkins University in Washington, said commercial satellite imagery from October and November showed construction of new propellant bunkers at the launch pad and engine test stand at the Sohae Satellite Launch Station.

It said these appeared to be “near completion, signifying what is likely the end of a three-year upgrade program.”

“That program is probably designed to support future activities related to the testing and launching of larger rockets,” the report said.

It said the upgrades meant North Korea would be ready to conduct such launches “by the first quarter of 2016 should the leadership in Pyongyang decide to do so.”

Pyongyang says its rocket launches are part of a legitimate space program aimed at putting satellites into orbit. In September its space agency said it was building a new satellite and readying it for launch.

Analysts say such a launch could signal an advance in extending the range of a ballistic missile or in increasing the weight of any weapons payload.

North Korea also said in September that its main nuclear complex was operating and it was working to improve the “quality and quantity” of weapons which it could use against the United States at “any time.”

On Tuesday, the United States added North Korea’s Strategic Rocket Force to its sanctions list for ties to weapons development and illicit finance activities.

It said the Strategic Rocket Force was responsible for a series of ballistic missile launches in 2014 including two medium-range Working-class missiles in March last year.