WASHINGTON -- A U.S. research institute says North Korea is conducting major construction at its main missile launch site, apparently to accommodate larger rockets and new mobile missiles.

The findings are based on analysis of Oct. 9 satellite imagery of the Sohae site where the North launched a long-range rocket into space last December in defiance of a U.N. ban on its use of ballistic missile technology.

U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said Monday a possible new launch pad for mobile missiles is being constructed and could be complete by mid-2014. A separate launch tower used for December's launch is also being upgraded, possibly to handle larger rockets.

It the latest sign North Korea is advancing its weapons programs despite saying it wants to resume disarmament negotiations.