WASHINGTON (Reuters) - The United States is very concerned that North Korea might not be able to be deterred, a senior Trump administration official said on Thursday, drawing a distinction between Washington s view of Pyongyang and its model for dealing with former Cold War foes. The official, speaking to reporters on condition of anonymity, also said there was a grave risk that North Korea might  miscalculate  the U.S. response to its behavior and warned Pyongyang not to  under-estimate American will to protect ourselves and our allies.  While U.S. experts are continuing to assess the North Korean nuclear test - its sixth and most powerful - the official that there was no information to contradict Pyongyang s assertion that it detonated a hydrogen bomb. 
U.S. unsure if North Korea can be deterred: Trump administration official