US secretary of state Rex Tillerson has said it might be necessary to make pre-emptive military moves against North Korea if the threat from its weapons programme reaches a level "we believe requires action".
Mr Tillerson was speaking at a news conference in Seoul, South Korea, after visiting the heavily militarised border with North Korea.
Asked about the possibility of using military force against the North, he said: "All of the options are on the table."
He said Washington does not want a military conflict, but if Pyongyang takes actions that threaten South Korean or US forces it would be met with an "appropriate response".
Past US administrations have considered military force because of North Korea's development of nuclear weapons, but the option has rarely been expressed so explicitly.
Mr Tillerson had earlier touched down in a helicopter at Camp Bonifas, a US base close to the Demilitarised Zone, created after the Korean War effectively ended in 1953.
He is the latest in a series of senior US officials to have their photos taken at the border, but it is the first trip by the new Trump administration's senior diplomat as he makes a tour of Japan, South Korea and China.
Speaking in Tokyo, Mr Tillerson had vowed a tougher strategy to confront North Korea's nuclear threat, but he offered no details about what would comprise the "different approach".
He pointedly noted that 20 years of "diplomatic and other efforts" had failed to dissuade the isolated communist government from developing its nuclear programme, which he called an "ever-escalating threat".