A U.S. policy of strategic patience with North Korea has ended, Secretary of State Rex Tillerson said in South Korea on Friday, adding that military action would be "on the table" if North Korea elevated the threat level.
Tillerson began his first Asian visit as secretary of state in Japan on Wednesday and will travel to China on Saturday with a main focus on finding a "new approach" on North Korea after what he described as two decades of failed efforts to denuclearize the insular nation.
"Let me be very clear: the policy of strategic patience has ended. We are exploring a new range of security and diplomatic measures. All options are on the table," Tillerson told a news conference in Seoul.
He said any North Korean actions that threatened the South would be met with "an appropriate response".
"If they elevate the threat of their weapons programme to a level that we believe requires action, that option is on the table," Tillerson said when asked about military action.
Tillerson also called on China to implement sanctions against North Korea and said there was no need for China to punish South Korea for deploying an advanced U.S. anti-missile system aimed at defending against North Korea.
China says the Terminal High Altitude Area Defense (THAAD) system's powerful radar is a threat to its security.
"We believe these actions are unnecessary and troubling," Tillerson said, referring to what South Korea sees as Chinese retaliation in the form of business restrictions in response to the deployment of the missile system.
"We also believe it is not the way for a regional power to help resolve what is a serious threat for everyone. So we hope China will alter its position on punishing South Korea."
"We hope they will work with us to eliminate the reason THAAD is required."
North Korea has conducted two nuclear tests and a series of missile launches since the beginning of last year.
Last week, it launched four more ballistic missiles and is working to develop nuclear-tipped missiles that can reach the United States.
South Korean Foreign Minister Yun Byung-se told the joint news conference the missile system was only intended to defend against North Korea, not any other country.