The system based in the US state of Alaska was switched from test to operational mode in the last two weeks, the Washington Times reported, but US officials have been non-committal about whether the US military would try to intercept a launch.
North Korea has reportedly began fuelling a Taepodong missile, which could be launched with days.
The possible test of missile has sparked strong diplomatic protests in East Asia and the United States. US Secretary of State Condoleezza Rice on Monday called it a 'provocative act' that would 'be taken with utmost seriousness.'
The limited US missile defence system is mainly based in Fort Greeley, Alaska, where there are nine interceptor missiles. There are two others at Vandenberg Air Force Base California.
Japan and the United States say a test launch would violate a 1999 and a 2002 agreement.