The work began in mid-year, and there are unlikely to be rocket launches at Sohae for the next six months while it is under way, the institute said. Its assessment includes analysis by Nick Hansen a retired intelligence expert who closely monitors developments in the North's weapons programs.
The construction at Sohae appears to have been continuing even as Pyongyang, with nudging from its ally China, has dialed down its threats against the U.S. and improved relations with South Korea.
Tensions escalated when North Korea in December managed for the first time to launch a satellite into orbit atop a long-range rocket, and then in February conducted an underground nuclear test explosion at a separate location.
Both actions defied U.N. Security Council resolutions and intensified concern that North Korea is moving toward its goal of building a bomb small enough to be fitted on an intercontinental missile.
South Korea's defense ministry declined to comment Friday on the reports of construction at Sohae. Wee Yong-sub, deputy spokesman at the Defense Ministry, said it closely monitors the area but cannot give details about intelligence matters.
The imagery shows six different construction activities, the institute says. The most notable feature measures about 65 yards by 40 yards and is located about 100 yards west of the existing launch pad. The institute says it's too early to identify the structure's exact purpose, but it could be a second, smaller launch pad intended for mobile missiles.
"Pyongyang clearly has plans to further develop its long-range rockets in the future," said Joel Wit, a former State Department official and editor of 38 North. "If it is indeed building a new flat launch pad, that would be a significant sign that the North is serious about testing and deploying new mobile missiles, possibly including an inter-continental ballistic missile spotted in recent military parades."
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Associated Press writer Youkyung Lee in Seoul, South Korea, contributed to this report.
