North Korea has begun fuelling a long-range rocket capable of sending a warhead into American territory by the weekend, US officials have said.
The secretive communist state claims it is planning to send a satellite into orbit between April 4 and 8.
But the United States, South Korea, and Japan have said the launch is a disguised test of the Taepodong-2 missile.
The fuelling signals that North Korea is in the final preparation stages for the launch, a report quotes US military officials as saying.
A launch would be the first big challenge for President Barack Obama in dealing with the North, whose efforts to build a nuclear arsenal have long plagued ties with Washington.
Speaking at the G20 summit in London, a US official said America would respond by raising the issue with the UN security council.
“The President made clear we are deeply concerned about the prospective missile launch by the North Koreans… There will be a reaction to it,” the official said.