U.N. nuclear inspectors have determined that traces of enriched uranium found in Iran came into the country on contaminated equipment bought through middlemen and dealers, some of whom were connected to Pakistan's nuclear black market, according to experts and diplomats working on the investigation.

The findings do not rule out the possibility that Iran may be concealing a weapons program, but they do lend support to the country's contention that it unknowingly imported tainted equipment.

U.S. officials have cited the residue as proof that Iran was enriching uranium or importing the material as part of a program to build a nuclear bomb, but the new findings could complicate U.S. efforts to muster international pressure on the Islamic republic over its nuclear program.

The uranium issue is expected to feature prominently when the International Atomic Energy Agency's 35-member board meets in Vienna next month to determine whether Tehran is violating international law.

The Bush administration, Iran and Europe's main powers are locked in a standoff in the face of evidence that Tehran has concealed elements of a nuclear program that the country insists is designed to produce peaceful energy.

Secretary of State Colin L. Powell said recently it was increasingly likely that Iran's behavior would have to be brought up with the U.N. Security Council. But France, Britain and others have been reluctant to do so without clear-cut proof of an Iranian nuclear weapons program.

On Monday, President Bush vowed to keep up the pressure on Iran but stopped short of threatening to use force.

IAEA inspectors have been scouring the country during the past 18 months to determine whether Iran is hiding anything. In earlier assessments, the agency said Iran's cooperation was weak, and it found inconsistencies in the country's reports about its nuclear program.

Yesterday, however, experts involved in the investigation said they now believe that particles of enriched uranium found in the country came from equipment sold by A.Q. Khan, the architect of Pakistan's nuclear weapons program, who was exposed earlier this year for supplying nuclear parts to Libya and Iran.

White House and State Department officials did not respond to calls for comment about the findings.

Inspectors, who found two levels of enriched uranium, said that particles enriched to 54 percent came directly from Pakistan's weapons program. They also said that particles enriched to 36 percent came from Russian equipment Pakistan may have bought secondhand or thirdhand years ago and which Khan later sold to Iran.

"The consensus has been for a while that the 36 percent enriched uranium had to have come from Russia because only Russia was producing that type of uranium," said Michael A. Levi, a science and technology fellow at the Brookings Institution. "The big question was always how the material made it from Russia to Iran," but Levi said contamination would explain that.

The IAEA is still trying to determine how and where Khan's network obtained the equipment, according to the experts, who spoke on the condition of anonymity.

The agency has been trying to keep the results of its investigation private until it can brief its board in a status report due next month. But some details were first revealed this week by the publication Jane's Defense Weekly.

"We expect to report any findings that we have on our analysis of the samples in our next report to the board in early September," said Melissa Fleming, spokeswoman for the IAEA.

Inspectors have interviewed more than a dozen middlemen and traders in an effort to learn about Khan's nuclear black market and how it supplied Iran. More questions remain regarding Iran's centrifuge program and whether it could work well enough to refine uranium to the 90 percent range necessary for creating a nuclear explosion.

France, Britain and Germany, hoping months ago to defuse tensions, had reached an agreement with Tehran on a suspension of suspect nuclear activities there in exchange for economic incentives. The deal unraveled in June when the three European nations and the IAEA board rebuked Iran for failing to fully cooperate with inspectors.

Two weeks ago, officials from all four countries met in Paris to try to salvage the deal, but neither side offered new incentives and instead traded blame for the deal's failure, European diplomats said.