Secretary of State Rex Tillerson said the option was "on the table" if the threat from the North's weapons programme reached a level requiring it.
During a visit to South Korea, he also said the US was exploring a range of new diplomatic and economic measures.
He also defended the deployment of a US missile system in South Korea.
The move has angered China, but South Korea and the US say the system is needed to defend against North Korea.
Mr Tillerson spoke shortly after visiting the demilitarised zone which divides the two Koreas.