Secretary of State Rex Tillerson is in Asia for his first foray into crisis management, and was to hold talks with South Korea's acting president Hwang Kyo-Ahn later, after China challenged him to come up with a new way to confront the North Korean nuclear stand-off.
Tillerson landed at Osan air base in South Korea from Japan and transferred to a Blackhawk helicopter for his trip to the DMZ, where he met the commander of the 28,000 US troops stationed in the South to defend the country.
He warned that past policies and punishments have had virtually no effect on Pyongyang's ambitions and that a new course was needed.
"I think it's important to recognise that the diplomatic and other efforts of the past 20 years to bring North Korea to a point of denuclearisation have failed," he said.
More From This Section
On Saturday Tillerson will head to China to press the North's key diplomatic protector and trade partner to back tougher sanctions -- but Beijing has been infuriated by the deployment of a US missile defence system to the South.
Four more test blasts have followed, two of them last year.
It has continued to defy the international community, even after two rounds of UN-backed sanctions, and last week test fired a salvo of missiles that fell in waters off Japan.
"In the face of the ever-escalating threat it is clear that a new approach is required," Tillerson said.
And he reiterated Washington's vow to back key regional allies Japan and South Korea in the event of attack.
US President Donald Trump stirred concern in the region during his White House campaign by suggesting allies like Japan and South Korea need to do more to defend themselves.