The Pentagon is designing a missile defense test program that would allow some of its elements, such as ground-based interceptors or airborne lasers, to be used in combat within a few years.

Bush administration officials said this would be done only in a national emergency such as a threat of missile attack from North Korea.

Pentagon officials, explaining the approach Thursday in testimony before the Senate Armed Services Committee, made clear that they intend to accelerate work on a deployable missile defense even at the cost of withdrawing from the 1972 Anti-Ballistic Missile Treaty banning national missile defenses.

The administration has set no date for deploying a defensive barrier against long-range ballistic missiles.

The officials encountered sharp questioning from some Democrats, including the committee's chairman, Sen. Carl Levin of Michigan, who declared themselves unwilling to support the administration's request for $8.3 billion for missile defense research and testing in 2002. That would be a 57 percent increase over this year's allocation.

Levin criticized Deputy Defense Secretary Paul Wolfowitz for failing to clarify for Congress whether missile defense tests planned for the budget year starting Oct. 1 would violate the ABM Treaty.

``I've yet to receive an answer,'' Levin told Wolfowitz. In his testimony, Wolfowitz said missile defense tests will ``bump up against'' treaty restrictions, and this probably will occur ``in months rather than in years.''

Wolfowitz said a number of missile test activities, including the start of construction in April 2002 on a new missile defense test facility, will raise questions about treaty compliance. He did not identify the site, but other officials said he was referring to Fort Greely and Kodiak Island in Alaska, where the Pentagon plans to build missile interceptor silos for test launches. Some preliminary work at Fort Greely is to start in August, with construction beginning in April, officials said.

Separately, in remarks at a missile defense conference on Capitol Hill, Defense Secretary Donald H. Rumsfeld said excavation and other ground-clearing work at Fort Greely gives no cause for concern.

``Everyone's hung up on tearing down some trees in Alaska, as though we're going to violate the treaty. We're not. Period. Full stop,'' Rumsfeld said.

Rumsfeld was asked when the missile defense testing program will come in conflict with the treaty.

``There's no way to know,'' he said. ``That's why they call it research and development — you're looking for things you don't know the answers to, and you don't know how fast they're going to go or how successful they're going to be.''

As the Pentagon looks for those answers, it is structuring its test program in a way that would allow some parts to be deployed as an interim defense, even before they are fully tested, Wolfowitz said.

``In an emergency we might, if appropriate, deploy test assets to defend against a rapidly emerging threat,'' he said. He noted that this was done with some weapons used during the 1991 Gulf War and 1999 Kosovo bombing campaign.

``But barring such an emergency, we need to consider the operational deployment of test assets very carefully, because such deployments can be disruptive, and can set back normal development programs,'' he said.

An administration paper, copies of which Levin's committee made public Thursday, is even more explicit about the intent to pursue testing in a way that would allow some elements to be used separately.

The paper cited two examples. One is a prototype airborne laser — a laser mounted on a modified Boeing 747— which is scheduled to attempt its first test shoot-down of a missile in 2003.

``If this test demonstrates the capability to intercept, the airborne laser prototype could be available to provide an emergency missile defense capability,'' the paper said.

Likewise, deployment of an interim ground-based missile defense system could be completed in Alaska as early as 2004 by upgrading existing radar and deploying missile interceptors drawn from the testing program, it said.

``A limited interim capability is warranted in light of existing and emerging near-term threats and the unpredictable nature of those threats,'' the paper said.

``Such capabilities, even if not fully mature, will provide more protection than we currently have — which today is no capability at all against longer-range missile able to strike American cities.''