The US secretary of state has arrived in South Korea with the aim of finding a "new approach" for relations with North Korea after what he described as two decades of failed efforts to denuclearise the reclusive state.
Rex Tillerson on Friday visited the Demilitarised Zone (DMZ), which divides the two Koreas in line with an armistice agreement signed in 1953, to gaze on the North for himself.
He said US military action against North Korea is an "option on the table", and warned the country to end its missile and nuclear programmes.
"The policy of strategic patience has ended," Tillerson said during his joint press conference with his South Korean counterpart Yun Byung-se.
Tillerson, a former oil executive, began his first Asian visit as secretary of state in Japan on Wednesday. He will travel to China from South Korea on Saturday.
He landed at Osan airbase 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.
"Certainly we do not want to, for things to get to military conflict," he said in Seoul, but "if they elevate the threat of their weapons programme to a level that we believe requires action, then that option is on the table."
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"Strategic patience" is the term given to the US policy under Barack Obama when the US ruled out engaging the North until it made a tangible commitment to denuclearisation, hoping that internal stresses would bring about change.
Previously, Tillerson had said in Tokyo that 20 years of diplomatic and other efforts, including a period when the US provided North Korea with $1.35bn in assistance "to take a different pathway", had come to nothing.
"In the face of this ever-escalating threat, it is clear that a different approach is required. Part of the purpose of my visit to the region is to exchange views on a new approach," he told a news conference, his first as secretary of state.
Al Jazeera's Rob McBride, reporting from Seoul, said that Tillerson stressed the need to forge closer cooperation between Asian allies of the US against North Korea.
"Tillerson said that US, Japan and South Korea should strengthen their trilateral partnership in the face of what they see as increasing aggression from North Korea," he said.
North Korea has a long-standing ambition to become a nuclear power and conducted its first underground atomic test in 2006, in the teeth of global opposition.
The country has continued to defy the international community for years, even after two rounds of UN-backed sanctions. It has conducted two nuclear tests and a series of missile launches since the beginning of last year.
Last week, North Korea launched four more ballistic missiles and is working to develop nuclear-tipped missiles that can reach the US.
The US has been pressing China to do more to rein in North Korea's nuclear and missile programmes, including imposing tougher sanctions on North Korea.
However, China has been infuriated by the deployment of a US missile defence system to the South. China says the system's radar is a threat to its security.
Source: Al Jazeera and news agencies