Hillary Clinton has warned North Korea relations with the US will not improve until it engages in dialogue with South Korea and ends its nuclear ambitions.
The US secretary of state was speaking in the South Korean capital, Seoul, during her first overseas trip as the top US diplomat.
Mrs Clinton also announced that Stephen Bosworth, a former ambassador, would be her new special envoy on North Korea.
Reports suggest the North may be about to test-fire a long-range missile.
Mrs Clinton said Pyongyang should follow through on its commitment to get rid of its nuclear programme, adding that it was important to get six-party talks on the issue back on track.
Mr Bosworth, who was US ambassador to South Korea from 1997 to 2000, will act as the American envoy to the six-party talks, which also involve Japan, China and Russia.