Secretary of State Rex Tillerson made the remark on a visit to South Korea, adding that the US was looking at all options to deal with the nuclear-armed state.
US Secretary of State Rex Tillerson on Friday said US strategic patience with North Korea has ended. Tillerson was speaking at a joint news conference in the capital Seoul with his South Korean counterpart Yun Byung-se.
Tillerson arrived in South Korea earlier on Friday for the second leg of an Asian tour focused on finding a "new approach" to the DPRK after what he described as two decades of failed efforts to denuclearise the isolated state.
Visit to DMZ
Tillerson earlier had his photo taken at the truce village of Panmunjom inside the Demilitarised Zone (DMZ), a cluster of blue huts where the armistice was signed that ended hostilities in the Korean War.
Tillerson also visited Camp Bonifas, a US base about 400 metres from the heavily fortified DMZ, meeting some of the 28,500 US troops stationed in South Korea.
Tillerson's tour of Japan, South Korea and China is his first as US secretary of state.
The former oil executive with no prior diplomatic experience was in Japan on Wednesday and will travel to China on Saturday.