President Bush announced plans yesterday to begin fielding a rudimentary system for defending the United States against missile attack, with an initial 10 land-based interceptors to go in Alaska and California in 2004.

The announcement put Bush on record for the first time committing to a specific deployment date and defining an initial system of missile defense. Pentagon officials stressed that the program would have very limited capability at first, aimed largely at knocking down any North Korean missiles. Many more years of development and testing will be necessary, they said, to provide a truly comprehensive U.S. anti-missile shield.

But the president's initiative represents a significant expansion over previous administration plans, which had foreseen simply a testing facility at Fort Greely, Alaska, by the time of the next presidential election.

Essentially, Bush has decided to turn the test facility into a missile defense site and to equip Vandenberg Air Force Base in California with missile interceptors to shoot down enemy warheads carried by long-range rockets. Under the plan, the Pentagon will also step up efforts to deploy ship-based and land-based interceptors to defend against shorter-range missiles.

To finance the project, defense officials said the administration would be seeking an extra $1.5 billion over the next two years, on top of $16 billion previously projected in that period for this and other missile defense projects.

Defense Secretary Donald H. Rumsfeld described the initial plan as "better than nothing," saying, "The reason I think it's important to start is because you have to put something in place and get knowledge about it and experience with it."

White House officials attributed the timing of Bush's announcement to preparation of the fiscal 2004 budget request to be submitted to Congress early next year. Despite recent setbacks in missile defense development and testing, Pentagon officials expressed confidence in their ability to build a workable if limited anti-missile shield. Rumsfeld presented the expanded missile defense plan to the White House several months ago, an administration official said.

Bush and senior aides have warned repeatedly about a growing missile threat from North Korea and Iran, and recent reports of a previously undisclosed North Korean effort to develop nuclear weapons have added to U.S.-North Korean tensions. But White House officials said Bush's decision was not linked to the latest North Korean developments.

In a statement, Bush said the new initiative "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 technologies and in light of changes in the threat."

Critics slammed the move as premature and likely to result in a waste of billions of dollars on inadequate technology. Sen. Carl M. Levin (D-Mich.), outgoing chairman of the Armed Services Committee, said Bush's action "violates common sense by determining to deploy systems before they have been tested and shown to work."

Lisbeth Gronlund, a physicist with the Union of Concerned Scientists, said, "The appropriate analogy is to the emperor's new clothes. They're in the middle of research and development and still lack many of the elements needed for the system. The danger also is that they're not going to do the other things they should be doing to deal with emerging threats, like negotiate with North Korea and get a handle on the spread of fissile material."

Yesterday's announcement came after a failed flight test last week in which the proposed interceptor failed to separate from its booster shortly after launch from an island in the central Pacific. But speaking to reporters at the Pentagon, Lt. Gen. Ronald T. Kadish, head of the Missile Defense Agency, cited successful intercepts in four previous attempts and a total record of five hits in eight attempts since testing began in 1999. He said these tests, along with others involving shorter range interceptors -- the Navy's Standard Missile-3 and the Army's upgraded Patriot system called PAC-3 -- have demonstrated the feasibility of using a missile to hit a missile, a concept known as "hit to kill."

"When you look across the board, we have made, I think, significant progress in our overall hit-to-kill technology," Kadish said. "And that's why we have gained the confidence that we could take this next, modest step."

The administration plan calls for putting six land-based interceptors at Fort Greely and four at Vandenberg in 2004. Ten more would be added at Fort Greely in 2005. These interceptors would consist of giant booster rockets designed to soar into space and release "kill vehicles" programmed to home in on long-range enemy warheads and obliterate them by the sheer force of collision.

In addition, as many as 20 smaller interceptors would be placed on three Navy ships for use against short- and medium-range missiles. Also included in the initial system outlined yesterday was a previously announced increase in the number of PAC-3 interceptors.

To detect and track enemy missiles, the Pentagon is building a high-resolution X-band radar that will be placed at sea and a new system of space satellites known as SBIRS-High. The plan also envisions employing as many as 15 Navy ships equipped with Aegis defense systems and SPY-1 radars. And the administration has officially requested use of early warning radars in Britain and Greenland that would be upgraded with advanced tracking capabilities. These radars, which are slated to be incorporated into the missile defense system in 2005, would be especially critical for monitoring missiles launched from the Middle East, officials said.

Bush took office intent on pursuing a workable anti-missile system, something that has eluded U.S. presidents for several decades. He launched the Pentagon on a broad program of experimentation to devise systems for combating enemy missiles in all phases of flight, from shortly after launch to the final seconds before impact.

With the demise of the 1972 Anti-Ballistic Missile Treaty last summer, military planners have been free to explore previously prohibited types of interceptors and radars. But in recent months, Rumsfeld and Kadish have moved to focus the program on the most promising missile defense elements, a move urged last summer by a Defense Science Board panel of senior advisers.

Initial plans to experiment with a space-based laser have been canceled.

A proposed airplane-mounted laser weapon also is experiencing delays. And the timeline for fielding some kind of weapon for intercepting long-range missiles shortly after launch in their "boost phase" has shifted to later in the decade.

Even the proposed system of land-based interceptors, while most advanced, continues to suffer from problems with development of a new booster rocket and concerns about the system's ability to deal with decoys.

"I think the way to think about the missile defense program is that it will be an evolutionary program; it will evolve over a period of time," Rumsfeld said at a Pentagon news conference yesterday. "When it finishes some day out there in the years ahead, it very likely will look quite different than it begins. And it very likely will have layers. And it very likely will involve a variety of different locations. And it will very likely involve the participation of a number of countries."

Defense Secretary Donald H. Rumsfeld: "It will be an evolutionary program . . . over a period of time."A missile interceptor is tested unsuccessfully over Pacific on Dec. 11.