A key piece of evidence in the Bush administration's case against Iraq is being challenged in a report by independent experts who question whether thousands of high-strength aluminum tubes recently sought by Iraq were intended for a secret nuclear weapons program.

The White House last week said attempts by Iraq to acquire the tubes point to a clandestine program to make enriched uranium for nuclear bombs. But the experts say in a new report that the evidence is ambiguous, and in some ways contradicts what is known about Iraq's past nuclear efforts.

The report, from the Institute for Science and International Security, also contends that the Bush administration is trying to quiet dissent among its own analysts over how to interpret the evidence. The report, a draft of which was obtained by The Washington Post, was authored by David Albright, a physicist who investigated Iraq's nuclear weapons program following the 1991 Persian Gulf War as a member of the International Atomic Energy Agency's inspection team. The institute, headquartered in Washington, is an independent group that studies nuclear and other security issues.

"By themselves, these attempted procurements are not evidence that Iraq is in possession of, or close to possessing, nuclear weapons," the report said. "They do not provide evidence that Iraq has an operating centrifuge plant or when such a plant could be operational."

The controversy stems from shipments to Iraq of specialized aluminum metal that were seized en route by governments allied with the United States. A U.S. intelligence official confirmed that at least two such shipments were seized within the past 14 months, although he declined to give details. The Associated Press, citing sources familiar with the shipments, reported that one originated in China and was intercepted in Jordan.

The shipments sparked concern among U.S. intelligence analysts because of the potential use of such tubes in centrifuges, fast-spinning machines used in making enriched uranium for nuclear bombs. High-strength, heat-resistant metals are needed for centrifuge casings as well as for the rotors, which turn at up to 1,000 rotations per minute.

There is no evidence that any of the tubes reached Iraq. But in its white paper on Iraq released to the United Nations last week, the Bush administration cited the seized shipments as evidence that Iraq is actively seeking to develop nuclear weapons. Bush's national security adviser, Condoleezza Rice, said in a televised interview that the tubes "are only really suited for nuclear weapons programs, centrifuge programs."

Since then, U.S. officials have acknowledged differing opinions within the U.S. intelligence community about possible uses for the tubes -- with some experts contending that a more plausible explanation was that the aluminum was meant to build launch tubes for Iraq's artillery rockets.

"But the majority view, held by senior officials here, is that they were most likely intended for gas centrifuges," one U.S. intelligence official said in an interview.

The new report questions that conclusion on several grounds, most of them technical. It says the seized tubes were made of a kind of aluminum that is ill-suited for welding. Other specifications of the imported metal are at odds with what is known about Iraq's previous attempts to build centrifuges. In fact, the report said, Iraq had largely abandoned aluminum for other materials, such as specialized steel and carbon fiber, in its centrifuges at the time its nuclear program was destroyed by allied bombers in the Gulf War.

According to Albright, government experts on nuclear technology who dissented from the Bush administration's view told him they were expected to remain silent. Several Energy Department officials familiar with the aluminum shipments declined to comment.

The Bush administration has said aluminum parts like these were to be used to make enriched uranium.