The US has said its policy of “strategic patience” with North Korea is over, and suggested it may decide to take pre-emptive military action.
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.
He arrived in South Korea from Japan, where he said that 20 years of efforts aimed at persuading North Korea to abandon its nuclear ambitions had failed.