Adm. Samuel Locklear told reporters that North Korea wants the world to believe that it has such a capability, so he's bound to take that threat seriously.
Over the past 18 months, North Korea has conducted a nuclear test, launched a long-range rocket into space and displayed at a military parade its road mobile KN-08 missile.
If operable, the KN-08 could potentially hit the U.S. But some experts say the missiles on show were mock-ups.
"Whether they are real or not, or whether they have the capability or not, (the) North Korean regime wants us to think they do and so we plan for that" to protect the U.S. and the region, Locklear said.
He said an unpredictable and nuclear-armed North Korea presents the most danger to the world within his area of military command, which stretches from waters off the west coast of the U.S. to the western border of India.
Locklear also stressed the importance of preventing territorial disputes in the Asia-Pacific that could be resolved through international law and compromise from becoming regional flashpoints.
China, in particular, has long-running disputes with many of its neighbors, including U.S. ally Japan. The two Asian nations have competing claims to remote islands in the East China Sea and that has stoked tensions.
China last week accused a Japanese naval and air patrol of disrupting a recent Chinese military drill that was using live ammunition. Japan denied disrupting the drill.
Locklear said it was not abnormal for Japan to have observed the Chinese exercises the way they did and Japan's action should not be viewed as provocative.
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