WASHINGTON - Major delays, cost overruns, and critical technological problems are plaguing a missile-defense system designed to protect the United States and Europe from an Iranian attack, Pentagon advisers and government investigators say about one of President Obama's top military programs.

The reports cast doubt on the shield, a politically sensitive issue at home and in relations with Russia. They say missile interceptors are running into production glitches, radars are underpowered, and sensors cannot distinguish between warheads and other objects.

A report by the Defense Science Board, an advisory group to the Defense Department, came out late last year but received little notice. While it concludes there are "no fundamental roadblocks" to the system, it points out big problems without saying how they can be fixed.

Board members declined repeated requests for comment. Outside experts, including the Pentagon's former chief weapons tester, Philip Coyle, say the issues raised in the report would require substantial and costly changes, if they can all be surmounted at all.

The second report, by Congress' nonpartisan Government Accountability Office, was released Friday.

Some Republicans say the reports support their view that the program was designed hastily to ease the concerns of Russia, which had objected to previous missile-defense plans by the Bush administration, with less regard to whether it would work. Ahead of November's election, Republicans are casting Obama as a weak leader, quick to capitulate to the demands of other nations.

"There is a political timeline and agenda that doesn't meet a scientific, development, and security timeline," said Rep. Michael Turner (R., Ohio), chairman of a House panel that oversees missile defense. "It does not appear that it can deliver the protection for U.S. homeland that this administration promised."

The administration insists the plans are on track.

Missile defense in Europe has been a nettlesome issue since the middle of last decade, when President George W. Bush announced plans to base long-range interceptors in central Europe as a defense against missiles from Iran. That infuriated Russia, which believed the program was intended to counter Moscow's intercontinental ballistic missiles and undermine its nuclear deterrent. Some Democrats also objected, complaining that the United States was gambling billions of dollars on questionable technology.

Soon after Obama took office in 2009, he revamped the program as he looked to improve relations with Moscow. His plans called for slower interceptors that could address Iran's medium-range missiles. The interceptors would be upgraded gradually over four phases, culminating in 2020 with newer versions, still in development, that the administration says will protect Europe and the United States. The early phases call for using Aegis radars on ships and a more powerful radar based in Turkey. Later phases call for moving Aegis radars to Romania and Poland.

The plans have gained momentum in Europe with the signing of basing agreements in Poland, Romania, and Turkey, as well as backing by NATO.

Obama maintains that his system would be more reliable than what had been planned by Bush because the new plan was based on tested technology.