The launch pad has been picked up on satellite images
North Korea has placed what is thought to be a long-range missile on a launch pad, Japanese and US officials say.
North Korea had already said it would send a satellite into orbit in early April, using a long-range missile.
The US, Japan and South Korea are concerned Pyongyang will test its Taepodong-2 long-range missile instead of launching a satellite.
US Secretary of State Hillary Clinton said that if North Korea launched a missile, there would be "consequences".
Japanese PM Taro Aso said plans were being made to shoot down any rocket that threatened to hit the country.
North Korea has said it plans to carry out the controversial launch between 4 and 8 April.
The Taepodong-2 missile is capable of reaching Alaska from the Musudan-ri base in Hwadae on North Korea's north-east coast.
It first tested the missile in July 2006, but it failed less than a minute after launch.
Speaking during a visit to Mexico, Mrs Clinton said the US would raise any missile launch with the UN, with an impact for stalled six-nation talks on ending North Korea's nuclear programme.
"We have made it very clear that the North Koreans pursue this pathway at a cost and with consequences to the six-party talks which we would like to see revived and moving forward as quickly as possible," she said.
Pyongyang has said that any UN sanctions imposed for the launch would mean the end of the six-nation talks.
Japan has suggested it could deploy a vessel equipped with missile interceptor technology to the Sea of Japan (East Sea) to shoot the rocket down.
Japan's security council is expected to meet this week to make preparations to shoot the missile down if it threatens to land on Japan.
Meanwhile, South Korea's chief negotiator to the six-nation talks, Wi Sung-lac, said a launch would trigger "counter-measures", without saying what those might be.
A satellite launch and a long-rang missile test would both use the Taepodong 2, analysts say.
Pyongyang has already said the rocket taking its satellite into orbit will cross over Japan, dropping booster stages to its east and west.
North Korea has warned the US, South Korea and Japan not to interfere with its launch.