The US Secretary of State, Condoleezza Rice, has warned North Korea of the consequences of testing a new ballistic missile that could, in theory, reach the United States.
North Korea is believed to be readying the launch of a Taepodong-2 missile that can carry a 1,000kg warhead up to 6,700km.
At that range, parts of Alaska in the US would be within reach, as would Asia and Russia.
At a news conference, Dr Rice said any move would amount to a 'dangerous provocation' that would be taken very seriously by the Bush administration.
Pentagon spokesman Bryan Whitman has not said whether the US would use its missile defence system against any North Korean missile launch.
However, he pointedly used the term 'launch' rather than 'test' to describe the preparations, and said North Korea's intentions were not clear.
In Seoul, a spokesman for the ruling Uri Party said the South Korean government had urged its neighbour not to launch the missile.
In 1998, North Korea fired a two-stage Taepodong-1 missile over Japan into the Pacific Ocean, causing international criticism.
It declared a moratorium on flight tests of long-range missiles in 1999 but said in 2005 that it would no longer keep to it.
