The United States will take every necessary measure to protect our citizens

Announcing the decision in a written statement, Mr Bush said the new "modest capabilities" - said to begin with the deployment of 10 ground-based interceptor rockets - "will add to America's security and serve as a starting point for improved and expanded capabilities later".

The decision comes days after the latest failure of an anti-missile test over the Pacific Ocean.

The programme has been gathering pace, with Mr Bush requesting the use of facilities in the UK and Denmark.

In June, the US withdrew from the 1972 Anti-Ballistic Missile Treaty it signed with the former Soviet Union, seeing it as constraining its ability to test new anti-missile defences.

Despite international criticism, Washington has insisted on developing the programme to defend against incoming missiles from what it calls "rogue states".

"The United States will take every necessary measure to protect our citizens against what is perhaps the greatest danger of all: The catastrophic harm that may result from hostile states or terrorist groups armed with weapons of mass destruction and the means to deliver them," Mr Bush said in his statement, read by White House spokesman Ari Fleischer.

"Today I am pleased to announce that we will take another important step in countering these threats by beginning to field missile defence capability to protect the United States as well as our friends and allies."

Details of the deployment at Fort Greeley, Alaska, are expected to be announced later, but media reports say the 10 interceptors supposed to be operational by 2004 are to be followed by another 10 a year or two later.

Mr Bush and his Defence Secretary Donald Rumsfeld say the proliferation of weapons of mass destruction and missile technology have sharply increased the need for such a defence against attack from "rogue states" - such as Iran, Iraq and North Korea - which has announced the resumption of its nuclear weapons programme.

The Fylingdales base in Britain has an advanced radar which could track missiles that could then, in theory, be shot down as they neared the US coast.

The Danish site, in Greenland, is also a radar station.

The missile shield is well beyond the planning stages

There have been eight tests of interceptor missiles so far - three have failed.

In June, the US started work on the construction of the Alaska test site for the missile defence system.

A set of silos is being built and is expected to be fully operational by 2007.

There has been considerable scepticism that the eventual goal, of a comprehensive shield capable of protecting the US from any missile attack, will ever be achievable.

Mr. Bush is expected to announce the decision today, with Defense Secretary Donald H. Rumsfeld and other defense officials then describing the details of the deployment plan.

The decision comes a year after the United States withdrew from the 1972 Anti-Ballistic Missile Treaty to allow more effective research and development of a system to shoot down long-range and short-range missiles. It also fulfills a presidential campaign promise Mr. Bush made in May 2000.

It marks the first time since the 1960s that the U.S. government will field an anti-missile system. President Reagan first announced the major shift toward strategic defenses and away from offensive nuclear missiles in 1983.

Until now, the Pentagon was investigating whether various methods of shooting down incoming missiles were feasible. Based on the past year of work on missile defenses, Mr. Bush decided to go forward with the limited system.

According to a senior administration official, the deployment plan calls for fielding 10 ground-based interceptor missiles at Fort Greeley, Alaska, by 2004 and an additional 10 interceptors by 2005 or 2006.

The system will provide the United States with a limited defense against long-range missile threats, primarily those posed by rogue states. Recent missile tests and U.S. intelligence reports have pinpointed North Korea, Iran and Iraq as the likeliest nations to pose such a threat.

The interceptors will be guided to targets by a global network of radars and sensors that will identify and track long-range missiles.

To deal with short-range and medium-range missiles, the Pentagon plans to deploy an updated version of the Navy's Standard Missile-3 on ships equipped with the Aegis battle management system.

American missile-defense plans have been criticized by Russia and China, most recently at a meeting earlier this month between Russian President Vladimir Putin and Chinese President Jiang Zemin.

The decision comes amid heightened tensions about North Korea, which announced recently that it is considering lifting its moratorium on missile flight tests. Pyongyang surprised U.S. intelligence agencies by flight-testing a long-range Taepo Dong missile in August 1998. North Korea also revealed that it had been secretly developing uranium-based nuclear weapons and would restart nuclear reactors that had been shut under a 1994 agreement.

Deputy Defense Secretary Paul Wolfowitz said in an Oct. 24 speech that "moving forward on missile defense, particularly by taking advantage of new technological opportunities, is an essential part of a strategy to provide the range of capabilities necessary to defend against the broad spectrum of new threats and challenges that we will confront in the 21st century."

Mr. Wolfowitz said the threat from short-range missiles "is here with us today" and that the threat from long-range missiles "may still be a few years away."

By withdrawing from the ABM Treaty, "we may now be in a position to be able to respond before that threat emerges," he said.

Preparatory construction at the first missile-defense site at Fort Greeley began in June, and other elements of the missile-defense test site will be built beginning in 2003.

In the past year, the Pentagon has begun conducting tests with short-range missile-defense systems that were prohibited by the ABM Treaty and has built and tested mobile and sea-based sensors that detect and track missiles.

"Our missile-defense program since 2001 has demonstrated that missile technology, in particular hit-to-kill technology, actually works," Mr. Wolfowitz said. "We actually can hit a bullet with a bullet."

A recent missile-defense test failed, however, on Dec. 11, when an interceptor rocket did not separate from its booster. Four earlier tests were successful.

Mr. Bush announced on Dec. 13, 2001, that the United States was withdrawing from the ABM Treaty.

"I have concluded the ABM Treaty hinders our government's ability to develop ways to protect our people from future terrorist or rogue state missile attacks," Mr. Bush said at the time.

Critics said the treaty withdrawal would lead to a new strategic-arms race, although the reaction from Russia and China has not gone beyond verbal criticism.

Mr. Wolfowitz said in his speech that the war against terrorism should not mean that the United States should stop developing missile defenses.

"It is clear that potential adversaries will pursue any means they can to exploit the vulnerabilities of a free society," he said.

"They will exploit the freedom and privacy rights in the West. They will exploit our reluctance to kill innocent civilians in time of war. And they most certainly will seek to exploit our near total vulnerability to ballistic missile and cruise missile attack."

As a candidate, Bush promised to build an anti-missile shield, and earlier this year he pulled out of an anti-ballistic missile treaty to advance the plan. Tuesday, he cited the Sept. 11, 2001 terrorist attacks as evidence that the country faces "unprecedented threats" and needs the anti-missile shield.

"When I came to office, I made a commitment to transform America's national security strategy and defense capabilites to meet the threats of the 21st century," Bush said in a prepared statement. "Today I am pleased to announce we will take another important step in countering these threats by beginning to field missile defense capabilities to protect the United States as well as our friends and allies."

He called the initial stage "modest," but said, "These capabilities will add to America's security and serve as a starting point for improved and expanded capabilities later as further progress is made in researching and developing missile defense technoloigies and in light of changes in the threat."

The plan calls for 10 ground-based interceptor missiles at Fort Greely, Alaska, by 2004 and an additional 10 interceptors by 2005 or 2006, defense officials said, speaking on condition of anonymity.

Bush said the "initial capabilities" will also include sea-based interceptors and sensors based on land, at sea and in space.

Defense Secretary Donald H. Rumsfeld and other defense officials scheduled briefings to explain details of the plan.

The Washington Times first reported the plan in Tuesday's editions.

The United States has asked to use a radar complex in northern England as part of a global missile defense shield, the British government said Tuesday.

Prime Minister Tony Blair's Downing Street office said no decision had been made on the written request to use the Royal Air Force base at Flyingdales in North Yorkshire.

American officials have also asked NATO member Denmark if it can upgrade a radar station at an American Air Force base in Greenland as part of the missile defense system, Prime Minister Anders Fogh Rasmussen said Tuesday. Greenland is semiautonomous territory with no say when it comes to foreign or defense policy.

Bush's announcement came six days after the latest test of the system failed when an interceptor rocket did not separate from its booster rocket and destroy a Minuteman II intercontinental ballistic missile as planned.

Three of eight tests of the ground-based system since 1999 have been judged successes by the military.

Once U.S. withdrawal from the 1972 Anti-Ballistic Missile treaty became official last summer, the Pentagon moved quickly to start work at Fort Greely on six underground silos for missile interceptors.

The treaty had barred such construction by either the United States or Russia. Bush gave Russia six-months notice of the withdrawal in December 2001.

The initial Bush plan is more limited than the Strategic Defense Initiative envisioned by President Ronald Reagan in 1983 that came to be known as "Star Wars."

Still, Bush expanded the program significantly from the ground-based plan pursued by President Bill Clinton by also ordering research and testing on sea-based and space-based systems.

Deputy Defense Secretary Paul Wolfowitz said in an October speech that "moving forward on missile defense, particularly by taking advantage of new technological opportunities, is an essential part of a strategy to provide the range of capabilities necessary to defend against the broad spectrum of new threats and challenges that we will confront in the 21st century."

The Pentagon has begun conducting tests with short-range missile-defense systems that were prohibited by the ABM Treaty and has built and tested mobile and sea-based sensors to track missiles.

"Our missile-defense program since 2001 has demonstrated that missile technology, in particular hit-to-kill technology, actually works," Wolfowitz said in his October speech. "We actually can hit a bullet with a bullet."

White House spokesman Ari Fleischer said the missile defense timing had nothing to do with North Korea's recent admission that it had a secret program to enrich uranium to make nuclear weapons. But, he noted, Bush cited North Korea as a threat when he promised during his campaign to build an anti-missile safety net.