The US would consider military action against North Korea if it was provoked, Secretary of State Rex Tillerson said.
Speaking in Seoul at a joint press conference with South Korean Foreign Minister Yun Byung-se, Tillerson said Washington's policy of "strategic patience" had ended.
"Obviously if North Korea takes actions that threatens the South Korean forces or our own forces, then that would be met with an appropriate response," he said.
The Secretary of State is on a three-country tour to the region amid heightened tensions on the Korean peninsula following recent North Korea missile launches, and fears it's preparing for another nuclear test.
In Tokyo, the new Secretary of State said that efforts over the past 20 years to prevent a nuclear North Korea had failed.
He said it was time for a new policy on North Korea, but released no details. It is clear that he sees Beijing, where he will fly to, as a key part of Washington's new approach.
A senior Chinese official told CNN this week that Beijing plans to present its own plan to Tillerson during his visit.