By Charles Scanlon
BBC News, Seoul
South Korea has warned North Korea not to conduct a nuclear test, following fresh warnings that the North is building up a nuclear arsenal.
Foreign Minister Ban Ki-moon said such a test would lead to further isolation of the communist state.
Mr Ban made his comments shortly after American press reports of activity at potential North Korean test sites.
Recent North Korean threats have rekindled fears of a dangerous confrontation over nuclear weapons.
The North Korean army said on Sunday that it would build up its nuclear arsenal in response to American aggression.
But Mr Ban has warned Pyongyang not to conduct a nuclear test, saying that exploding a bomb would further isolate the country and endanger its future.
The chief American negotiator handling the standoff, Christopher Hill, is back in the region, amid signs that time could soon be running out for a diplomatic solution.
North Korea is refusing to return to six-party talks, which have not taken place since June last year.
The US has warned it could go to the UN Security Council to ask for sanctions if the North continues to hold out.
South Korea and China have opposed such a move, fearing a potentially violent response from Pyongyang, but Seoul may feel obliged to acquiesce if North Korea ends the ambiguity of its capabilities and carries out a nuclear test.