U.S. Secretary of State Rex Tillerson says military action against North Korea is "on the table" if the country keeps developing nuclear weapons.
"Let me be very clear: The policy of strategic patience has ended," Tillerson said.
During a press conference in South Korea, Tillerson told reporters the U.S. doesn't necessarily want a military conflict with the North.
But he emphasized if North Korea does anything to threaten the South or the U.S., there will be "an appropriate response."
Tillerson's warning comes a day after his press conference in Tokyo, where he called for a new approach to handling North Korea.
"The diplomatic and other efforts of the past 20 years to bring North Korea to a point of denuclearization have failed. So we have 20 years of a failed approach," Tillerson said.
Earlier this month, North Korea fired four ballistic missiles into the Sea of Japan, and experts say they think North Korea's missile program is evolving quickly.
Several U.S. officials told CNN the U.S. Defense Department is watching for another round of missile and nuclear weapons testing, which is expected soon.