North Korea has developed the capability to pair a nuclear warhead with a missile and launch it, but has not mastered bringing the weapon back from space and onto a target, a senior US defense official said Thursday.

"They have the capability right now to be able to deliver a nuclear weapon, they are just not sure about re-entry," the official said. 

"They are not sure of the re-entry capability for a strategic strike, so they are endeavoring to try and overcome that," the official added.