WASHINGTON - The multibillion-dollar U.S. ballistic missile shield due to start operating by Sept. 30 appears incapable of shooting down any incoming warheads, an independent scientists' group said on Thursday.

A technical analysis found "no basis for believing the system will have any capability to defend against a real attack," the Union of Concerned Scientists said in a 76-page report titled Technical Realities.

The Pentagon's Missile Defense Agency rejected the report.

"It will provide a defense against incoming missiles for the first time in this country's history," said Richard Lehner, an agency spokesman.

The Pentagon's initial deployment involves 10 interceptor missiles in silos in Alaska and California. It is designed to protect all 50 U.S. states against a limited strike from North Korean missiles that could be tipped with nuclear, chemical or biological warheads.

Boeing Co. is assembling the shield, which would use the interceptors to launch "kill vehicles" meant to pulverize targets in the mid-course of their flight paths, outside the Earth's atmosphere. Using infrared sensors, the vehicles would search the chill of space for the warheads. So far, the interceptors have scored hits five times in eight highly controlled tests.

The Missile Defense Agency "appears to be picking numbers out of thin air," the report said of past Pentagon assertions of a high probability of shooting down targets.

"There is no data to justify such an assumption," added the scientists' group, which is based in Cambridge, Massachusetts. Its findings dovetailed with an audit last month by Congress's General Accounting Office that said the system's effectiveness would be "largely unproven" when the initial capability goes on alert.

DANGEROUS POLICY IMPLICATIONS

Even unsophisticated countermeasures that could be mounted by countries such as North Korea remain an unsolved problem for midcourse defenses against long-range missiles, the scientists' report said.

Balloon decoys could be given the same infrared signature as a warhead by painting their surfaces, it said. The project could also be confused by sealing the warhead in a large balloon so the kill vehicle could not determine its exact location or tethering several balloons to it.

Overstating the defensive capabilities of the ground-based defense is dangerous, the group said.

"If the president is told that the system could reliably defend against a North Korean ballistic missile attack, he might be willing to accept more risks when making policy and military decisions," the report said.

"All indications are that it would not work," added Lisbeth Gronlund, a physicist who is a co-author of the report and co-director of the group's global security program.

"And the administration's statements that it will be highly effective are irresponsible nonsense," she added in a telephone interview.

Overall, the Pentagon estimates it will need $53 billion in the next five years to develop, field and upgrade a multilayered shield also involving systems based at sea, aboard modified Boeing 747 aircraft and in space.

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