South Korean Foreign Minister Ban Ki-moon has called on North Korea not to carry out reported plans to test a long-range missile and to return to six-party talks aimed at dismantling its nuclear programmes.
Clouds and storms today closed on a site where North Korea may be preparing to test a missile, potentially delaying a flight.
In a speech to the United Nations-backed Conference on Disarmament, Mr Ban said that the North Korean nuclear issue was one of the most serious challenges facing northeast Asia.
"My government is deeply concerned that a launch of a long-range missile by North Korea would have serious negative repercussions for stability on the Korean peninsula and northeast Asia as well as for international efforts against proliferation of weapons of mass destruction," he said.
"North Korea is strongly urged to refrain from taking such a negative step," he said in a text prepared for delivery to the 65-member state forum in Geneva.
The North should return to the six-party talks without preconditions, said Mr Ban, who is a candidate to take over from Kofi Annan as United Nations Secretary-General at the end of the year.
The last round of the nuclear talks among the two Koreas, China, Japan, Russia and the United States was held in November 2005.
US officials say evidence such as satellite pictures suggest North Korea may have finished fuelling a Taepodong-2 ballistic missile for a test launch - which Washington, Seoul and Tokyo have said would present a grave threat to regional security.
Some experts say such a missile could reach as far as Alaska.
The United States and Japan have promised harsh action if the test flight goes ahead.
South Korea's weather agency forecast overcast skies and storms today in North Hamgyong province, where North Korea has a launch site and said this should be the pattern for the rest of the week as a storm front moves through.
Analysts say clouds and storms would make it difficult for North Korea to track a missile once in flight, decreasing the likelihood of a launch.