US President Mr George Bush has ordered the military to begin deploying a national missile defence system with land- and sea-based interceptor rockets to be operational starting in 2004.
The controversial decision to take what Mr Bush called a "modest" first step toward a more sophisticated defence in future years came despite last week's failure of a major anti-missile test over the Pacific Ocean.
Defence officials planned to deploy 10 ground-based interceptors - at least six at Fort Greely, Alaska, and perhaps four at Vandenberg Air Force Base, California - in 2004 and an additional 10 ground interceptors in 2005.
The ground interceptors would apparently protect US targets against long-range attack. Other interceptors for use against shorter-range missiles would be deployed aboard US Aegis warships beginning in 2004 and those could help defend allies in other regions of the world, officials said.
Erecting such a defence shield is the Pentagon's single most expensive development program and critics say it could cost hundreds of billions of dollars over coming decades.
Air Force Lt. Gen. Ronald Kadish, head of the Pentagon's missile defence effort, said the US Defence Department intended to spend more than $17.5 billion over the next two years to field the system. The department spent $8 billion a year in the previous two years.
In a statement, Bush said the move was "to protect our citizens against what is perhaps the greatest danger of all - catastrophic harm that may result from hostile states or terrorist groups armed with weapons of mass destruction and the means to deliver them."