America's chief envoy to North Korea has warned that the US will not wait forever for Pyongyang to return to six party talks on its nuclear programme.
Christopher Hill is in Beijing for talks
Christopher Hill accused the isolated North of stalling over the issue.
Mr Hill is currently in Beijing for talks with Chinese officials on the latest efforts to persuade North Korea to return to the negotiating table.
Tensions have risen recently, amid reports that Pyongyang might be preparing for a nuclear test.
Pyongyang has already said it possesses nuclear weapons, and is intending to bolster what it calls its nuclear deterrent.
Washington recently warned that it might go to the UN Security Council for possible sanctions if North Korea continued to refuse to negotiate.
"We've had the six-party process going on for some two years," Mr Hill told the BBC.
The Yongbyon reactor has been at the centre of the nuclear row
"Last June, we had a pretty comprehensive proposal we'd put on the table. The North Koreans said they'd study it and get back to us... and we're still waiting."
He said that while Pyongyang had offered various excuses for the delay, "the problem is that we can't go on waiting forever".
Concerns about the isolated North have risen recently, after reports that it had shut down its nuclear reactor at Yongbyon - which could allow it to reprocess weapons-grade plutonium.
"We are concerned that they're busily going ahead with their nuclear programmes," Mr Hill said - a move he described as unacceptable.
American newspaper reports suggest there has also been increased activity at North Korean sites where a nuclear test could be carried out.
Mr Hill is currently on a tour of Asia to talk with officials about the ongoing stand-off with North Korea.
On Monday he visited Seoul for discussions with South Korean government leaders.
South Korean Foreign Minister Ban Ki-moon said a nuclear test would be a "reckless step" that would endanger North Korea's future, and "further deepen the North's own isolation".
During his trip to China, North Korea's closest ally, Mr Hill will hear about Beijing's latest efforts to persuade Pyongyang to fall into line.
China has already hosted three rounds of six-way talks - which comprised delegates from the US, Japan, Russia, China and North and South Korea.
A fourth round of talks was due to be held last year, but did not take place because of Pyongyang's demand for concessions from the US and an end to what it called Washington's hostile policy.
After his trip to China, Mr Hill is due to travel to Japan on Wednesday for further discussions.