For years, South Korea and the international community have tried to coax North Korea into giving up its nuclear and missile program.
The 37-year-old formally rose to power on Dec 30, 2011, 13 days after his father Kim Jong-il died.
The leader is flaunting his power while at the same time refraining from presenting himself as a deity to his people.
Sullivan explained the Biden administration pursued striking a balance between the policy implemented by previous administrations.
A Pentagon spokesman also said the US has no hostile policy toward the North.
The US declared sanctions and visa bans on human rights grounds against officials and entities in eight countries, including North Korea.
Suspected nuclear activities were previously observed at the laboratory from mid-February to early July.
Kim repeatedly underscored the “importance of military education in the current situation".
Asia Power Index shows N. Korea’s military capability outstrips Japan and Australia.
North Korea is set to mark the 10th anniversary of leader Kim Jong-un's rise to power later this month.