The findings are based on satellite photos of the west coast site of Sohae, analyzed by the US-Korea Institute at Johns Hopkins School of Advanced International Studies.
North Korea blasted a rocket into space from site a year-and-half-ago, and the photos indicate it has made significant, recent progress upgrading facilities at Sohae to handle bigger rockets.
A launch tower has been expanded to handle rockets 20 meters higher than the 30-meter-long Unha-3 that blasted off in December 2012.
Adm Samuel Locklear, commander of US forces in the Pacific, told reporters today he believes North Korea has continued to make "steady progress" in both its missile technology and nuclear capability. He said he has to "plan for the worst" of what North Korea says it has or demonstrates it might have.
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A major construction program has been underway at Sohae since mid-2013, according the institute, which is publishing the findings on its website, 38 North.
In addition to the higher launch tower, a new wider road has been completed at Sohae that could carry larger rockets to the launch pad, and an existing rail line is being built that could also perform that function by next year, 38 North says.
Perhaps of greatest concern to the US are the indications Pyongyang recently conducted its fourth test this year of an engine that could be used for a mobile intercontinental missile known as the KN-08.