Secretary of State Rex Tillerson says it may be necessary to take pre-emptive military action against North Korea if the threat from its weapons program reaches a level "that we believe requires action."
Tillerson spoke at a news conference in Seoul after visiting the heavily militarised border with North Korea on Friday.
Asked about the possibility of using military force against the North, Tillerson said, "all of the options are on the table."
He said the US does not want a military conflict, but if North Korea takes actions that threaten South Korean or US forces it would be met with an "appropriate response."
Past US administrations have considered military force because of North Korea's development of nuclear weapons, but rarely has that option been expressed so explicitly.
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Earlier he warned that the policy of "strategic patience" with North Korea was over.
