US Secretary of State Condoleezza Rice has said Washington will not wait for ever for North Korea to restart talks on its nuclear ambitions.
North Korea says it has nuclear weapons
Speaking in South Korea during an Asian tour, she again urged the North to return to the negotiating table.
Earlier Ms Rice visited a command centre in Seoul as US and South Korean troops began joint exercises.
Pyongyang says it possesses nuclear weapons, and that they are aimed at countering threats from the US.
Since 2002, three rounds of discussions involving the US, Russia, the two Koreas, Japan and China have sought to ease tensions on the peninsula, with little success.
On Sunday Ms Rice said the US and regional powers would intensify efforts to restart the talks.
But she added: "We need to resolve this issue, it cannot go on forever."
She was speaking at a news conference with South Korean Foreign Minister Ban Ki-moon.
Pyongyang says Ms Rice must apologise for her earlier description of the country as an "outpost of tyranny", before it considers resuming six-nation talks.
While in Japan on Saturday, the secretary of state re-affirmed that the US had no intention of attacking North Korea.
Washington was prepared to offer security assurances to Pyongyang if it agreed to end its nuclear programme, she added.
Ms Rice is now travelling on to China.
The BBC's Rupert Wingfield-Hayes in Beijing says her talks there are likely to be some of the most difficult of her whole trip.
The Bush administration wants China to do more to force North Korea to halt its nuclear programme and return to negotiations.
But China insists it has limited influence and has called on the US to be more flexible.
While in Seoul, Ms Rice also expressed concern about rising Chinese military power and criticised the European Union over its plans to lift an arms embargo.