Libya has offered conflicting information about whether North Korea or Pakistan supplied uranium for its nuclear weapons program and has been unable to account for some equipment that could be used to make a bomb, according to the International Atomic Energy Agency, which released a report yesterday on its investigation of Libya and the nuclear black market that supplied it.

IAEA inspectors said efforts to resolve one of the biggest mysteries about Libya's program were complicated by statements from one Libyan, who said the uranium came from North Korea, and from another who pointed the finger at Abdul Qadeer Khan, Pakistan's top nuclear scientist. Khan supplied much of Libya's nuclear infrastructure.

The IAEA report made it clear that some countries are cooperating with its investigation. But the report reflects the difficulty its inspectors are having as they try to unravel the Pakistani black market that supplied Libya and Iran, and to understand the extent of international trafficking in nuclear materials.

"We've had conflicting reports and we can't nail it down," one IAEA official said, referring to the competing claims about Libyan suppliers. "But if North Korea is another player in the black market, then things are much worse than we know." The official spoke on the condition of anonymity because of the matter's sensitivity.

In Washington, a Bush administration official familiar with the report said the source for the North Korea claim was credible, but there was nothing else to corroborate the story.

North Korea is not believed to have the capability to supply the type of uranium found in Libya, and there has been no firm evidence that it provides nuclear materials to other countries.

Throughout the IAEA report, which was written ahead of the agency's Sept. 13 board meeting, Libya is praised for providing inspectors with access to facilities and responses to inquiries. But the report notes that Libya has failed to account for sophisticated enrichment technology that could have been stolen, hidden or lost, and also notes that some of Libya's responses have not been borne out by test results and soil samples.

Despite Libya's commitment to the United States and Britain to come clean about its weapons programs, "there are gaping holes in this investigation," another IAEA official said.

"Much of what the Libyans have told us appears to be consistent with our findings, but the black market is still murky enough that we're not closing any doors right now," the official said. Libya is hoping that the IAEA's board will agree next month that the country no longer requires special inspections.

The IAEA has been active in Libya since the country's leader, Moammar Gaddafi, agreed to give up his biological, chemical and nuclear weapons programs in December 2003. That concession was part of a deal that ended years of sanctions against the country for its role in the 1988 bombing of a Pan Am jet that killed 270 people in Lockerbie, Scotland.

The White House often cites Libya's decision as evidence of progress in its efforts to keep weapons of mass destruction out of the hands of terrorists. On Monday, President Bush said while campaigning that Gaddafi had "heard a clear message and voluntarily got rid of his weapons of mass destruction program."

Libya's decision exposed Khan, and the IAEA believes that he and a network of middlemen in 20 countries supplied Libya and Iran with equipment and technology for enriching uranium.

Khan also is expected to feature prominently in an IAEA report due this week on Iran, which maintains that its equipment is for use in a program designed to produce energy, not weapons. Khan, the father of Pakistan's nuclear program, is considered a national hero at home. Despite his activities, he was pardoned by Pakistani President Pervez Musharraf and lives under government protection.

The agency is conducting forensic analysis of warhead designs Khan gave Libya, in an effort to determine whether the drawings were copied or shared with other countries. The designs were Chinese in origin, obtained by Pakistan and then sold by Khan.

The report also expressed frustration with the level of cooperation by Pakistan. In a veiled reference to Khan and Pakistan, the agency wrote that its ability "to derive a credible assessment . . . would benefit greatly from the provision of additional information, including from the provider of the weapons design."

The agency also noted that Pakistan has refused to allow inspectors to take samples at Pakistani laboratories that could help confirm where Libya and Iran got their nuclear materials. The Pakistanis have insisted on conducting their own tests, without outside observers, and then sharing data with the IAEA.

"This investigation is continuing but can only be completed if the agency is permitted to take independent swipe samples at locations where the enriched uranium contamination may have originated," the IAEA wrote.

One Libyan said Abdul Qadeer Khan provided uranium to his country.