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.
The secretary of state is due in Beijing later on Friday, the final stop of her inaugural Asian tour, which has also included stops in Indonesia and Japan.
South Korea's prosperity and democracy stood in stark contrast to "the tyranny and poverty across the border to the North", Mrs Clinton said during a press conference with South Korean Foreign Minister Yu Myung-hwan.
"North Korea is not going to get a different relationship with the United States while insulting and refusing dialogue with the Republic of Korea," she said.
Her visit to the region comes amid speculation over the health of North Korean leader Kim-Jong-il, after reports he had a stroke last year.
Mr Kim is not known to have named his successor and Western diplomats fear any leadership crisis could further raise tensions on the peninsula.
Referring to speculation Pyongyang was preparing to test-fire a long-range missile, Mrs Clinton said the US viewed any such tests as provocative.
"We don't comment on intelligence matters but it is clear that under the United Nations that under the United Nations Security Council Resolution 1718, North Korea is required to suspend all activities related to its ballistic programme.
"The North should refrain from violating this resolution and also from any and all provocative actions that could harm the six party talks and aggravate tensions in the region."
BBC state department correspondent Kim Ghattas in Seoul says Mrs Clinton's words were a stern warning to Pyongyang, making clear that while the new administration may be ready to explore new strategies, there will be no softening of tone.
She also praised South Korea for what she described as its calm resolve in the face of provocations from the North, our correspondent adds.
Hours before Mrs Clinton arrived in Seoul, North Korea issued the latest in a series of warnings to its southern neighbour, stating that its troops were "fully ready" for war.
The North's sabre-rattling over the past few weeks is seen as an attempt to grab the attention of the Obama administration and improve its bargaining position when the talks eventually resume, our correspondent says.
Mrs Clinton arrived in Seoul from Indonesia - the world's most populous Muslim country - where she said the US was seeking a new kind of dialogue with the Muslim world.