More than ever, Iraqi President Saddam Hussein remains "determined to get his hands on a nuclear bomb," Secretary of State Colin L. Powell told the United Nations yesterday. But he failed to settle a dispute over whether an intercepted batch of aluminum tubes constitutes proof of Iraq's nuclear ambitions.

Iraq's attempt to import tens of thousands of high-strength aluminum tubes over the past two years is at the core of the Bush administration's case against the Iraqi leader.

Powell yesterday sought to bolster the argument that Iraq intended to use the tubes to make enriched uranium, not artillery rockets, as Iraq has claimed. During its well-documented nuclear weapons program in the 1980s, Iraq used imported aluminum tubes to build gas centrifuges -- fast-spinning machines used to enrich uranium.

"There is no doubt in my mind," Powell said, "that these illicit procurement efforts show that Saddam Hussein is very much focused on putting in place the key missing piece from his nuclear weapons program, the ability to produce fissile material."

Powell released a few additional details about the attempted acquisition, revealing that Iraqi officials had ordered tubes with unusually exacting specifications and high tolerances for heat and stress.

Over a period of months, the Iraqi invoices called for "higher and higher levels of specification," including metallic coatings on inner and outer surfaces, he said.

Powell argued that Iraq would not have gone to such trouble and expense if the tubes were intended for ordinary rockets that "would soon be blown to shrapnel."

"It strikes me as quite odd that these tubes are manufactured to a tolerance that far exceeds U.S. requirements for comparable rockets," Powell said. "Maybe Iraqis just manufacture their conventional weapons to a higher standard than we do, but I don't think so."

Powell's arguments were a direct challenge to the U.N. nuclear watchdog, the International Atomic Energy Agency, which after weeks of investigation concluded that the tubes were likely intended for Iraq's artillery rocket program. In a report to the U.N. Security Council last month, IAEA Director General Mohamed ElBaradei said the tubes were not suitable for uranium enrichment without significant modification.

Other sources said the tubes exactly matched the dimensions of Iraq's existing arsenal of 81mm artillery rockets. Iraq had ordered the same type of aluminum tubes in the 1980s to replenish its rocket stockpile.

U.S. and international nuclear experts have been divided about the nature of Iraq's new -- and ultimately unsuccessful -- attempts to purchase the aluminum parts. Powell's additional evidence appeared to have only widened the disagreement.

One former scientist in Iraq's nuclear program called Powell's arguments "persuasive." Khidhir Hamza, a physicist who defected to the United States in 1994, acknowledged that the tubes sought by Iraq were not ideal for centrifuges, but he suggested that Iraq may have tried to throw off U.S. intelligence agents and disguise its true intentions. After extensive machine-tooling of the tubes, Iraq could have used them to make enough uranium for up to two bombs a year, Hamza said.

"Of course Iraq would not order cylinders with exact specifications for centrifuges, because such tubes would never have been shipped," Hamza said. "This is a standard Iraqi ploy."

But another expert familiar with Iraq's previous nuclear program said it was also typical of Iraq to "over-specify" when ordering common weapons materials and parts. David Albright, president of the Institute for Science and International Security, said Iraq's use of increasingly higher standards in ordering aluminum tubes stemmed from technical problems with its rocket production: Rockets made of lesser grades of aluminum were getting stuck in the launcher tubes.

"The tubes are an important indicator, but they are not specific to centrifuges," Albright said. "I would not feel comfortable arguing on this basis that Iraq has a nuclear program -- even though I personally believe it does."

The IAEA's ElBaradei, who is ultimately responsible for determining the truth about Iraq's nuclear program, declined to pass judgment on Powell's analysis. "We have listened to Secretary Powell's presentation," he said, "and will factor it into our analysis."