A rudimentary missile defense system set to be installed in Alaska next year would be able to intercept and destroy North Korean nuclear warheads fired at U.S. cities, a top Pentagon official testified yesterday.

In an appearance on Capitol Hill, Undersecretary of Defense Edward C. "Pete" Aldridge Jr. said the system, expected to be operational by the end of 2004, would be "90 percent" effective in intercepting missiles fired from the Korean peninsula.

Aldridge's surprising claim -- which lawmakers immediately challenged -- could add to tensions between Washington and Pyongyang. Relations have been near crisis since North Korea admitted last year that it had restarted its nuclear program.

Aldridge, the top weapons-acquisition official at the Pentagon, said the capability will give President Bush "many more options" in confronting Pyongyang. He did not elaborate.

Appearing before the Senate Armed Services Committee, Aldridge and other Pentagon officials also backed away from a Defense Department proposal that would have exempted the missile defense system from ordinary weapons-testing requirements.

The hearing marked the latest in a long series of heated exchanges on Capitol Hill over the missile defense system, a controversial program first pushed by President Reagan in the 1980s.

For years the program was criticized as too costly, unlikely to ever be effective, and destined to alienate allies and other nations opposed to the effort.

But the program has been a top priority of the Bush administration, and it is poised for billions of dollars in additional funding.

Aldridge's comments were greeted with disbelief from lawmakers and missile defense experts, who noted that the system has had meager success so far intercepting missiles even in highly controlled tests.

Sen. Carl M. Levin (Mich.), the ranking Democrat on the Armed Services Committee, said Aldridge's claims were "exaggerated" and out of line with previous estimates the Pentagon has provided in classified documents.

"You better go back and check the classified numbers," he said. "I think you'll want to correct the record after you read the classified numbers."

Sen. Evan Bayh (D-Ind.) also was incredulous and pressed Aldridge on whether he would make the same claim to the president if faced with a North Korean attack.

Confronted with "the possibility of the North Koreans hitting Los Angeles or San Francisco with a nuclear warhead, you are advising [the president] that we would have a 90 percent chance of taking that down?" Bayh asked. "If millions of lives depend on it, that's your answer?"

Aldridge replied, "Yes, sir."

Experts also questioned the claim.

Philip Coyle III, who was the chief Pentagon weapons inspector during the Clinton administration, noted that, in tests over the Pacific Ocean in recent years, interceptors have struck American-fired target missiles five times in eight tries.

In those tests, the target missiles were fitted with beacons to make them easier for interceptors to find. And the interceptors were preprogrammed with data on the target missiles' intended paths.

"North Korea wouldn't send a missile with a beacon on it," Coyle said.

As part of the next phase in the development of the missile defense system, the Pentagon is planning to install 10 interceptors in silos near Fort Greeley, Alaska, about 100 miles south of Fairbanks.

Doing so would appear to violate laws requiring new weapons system to be subject to operational testing before they are deployed -- laws designed to guard against the Pentagon putting expensive weapons systems in place only to find they don't work.

But Defense Secretary Donald H. Rumsfeld has argued that the missile defense system is such a high national-security priority that its testing and deployment should be simultaneous.

In testimony last month, he said North Koreans "very likely do have a two-stage [rocket] with a kick motor capability which could reach the United States." Intelligence reports have said North Korea is developing a three-stage rocket that could reach the entire U.S. mainland.

By mid-decade, Iran also could have a missile capable of reaching the United States, said J.D. Crouch, assistant secretary of defense for international security.