U.N. inspectors have not uncovered definitive evidence that Iran has a clandestine nuclear weapons program, but they have been unable to clear up a series of suspicions and unanswered questions surrounding Tehran's activities, according to U.S. and Western diplomats who have been briefed on an upcoming International Atomic Energy Agency report.

The United States, which believes Iran could be three to five years away from completing a bomb in secret, shared intelligence tips with the IAEA in June, according to the diplomats, who agreed to discuss the classified information on the condition of anonymity.

Some of that information, including communications intercepts and satellite imagery, was followed up on by IAEA inspectors but did not lead to any discoveries. Several tips have yet to be fully explored and others were considered too vague, the diplomats said.

The IAEA's mixed report, which officials said will note improved cooperation from the Iranians in recent months, comes as the Bush administration is trying to make a case for stepping up pressure on the Islamic republic. Experts said the report's findings -- which could be made public as early as today -- will be a critical factor in that effort.

The United States lacks enough support from the IAEA's other 34 members to send the issue to the U.N. Security Council, which can assess international sanctions against Iran. But U.S. officials said they would continue to push for such an outcome.

Iran insists its program is peaceful and aimed at producing a stable energy source and has said it wants to avoid a showdown at the United Nations. Both sides are hoping the report will help sway board opinions.

"The report doesn't exonerate Iran, but it's not going to help get Iran to the Security Council, either," said David Albright, president of the Institute for Science and International Security, which recently revealed satellite photos of suspect sites that Iran leveled this year.

The IAEA is investigating those sites and other lines of inquiry, including Iran's relationship to a Pakistani scientist who ran a nuclear black market that was exposed last year.

Information regarding Abdul Qadeer Khan's shadowy network is expected to feature prominently in the IAEA's report. Inspectors recently determined that some samples of uranium found in Iran were brought in on contaminated equipment Tehran bought from the network. That finding help support Iran's earlier contention that the presence of uranium was caused by the tainted equipment, not by a secret program to enrich uranium for a bomb.

Among the areas still being pursued are questions related to Iran's potential nuclear weaponization activities, an inventory of equipment and materials purchases Iran made on the black market and the possibility that it could have obtained weapons designs similar to the ones Libya bought from the same network.

U.S. officials said they will emphasize those issues when it presses the IAEA board at a Sept. 13 meeting in Vienna to increase pressure on Tehran by referring the matter to the Security Council.

But they said much will depend on the position taken by Europe's three main powers -- France, Britain and Germany. Officials there have become deeply frustrated with Iran but are reluctant to take the matter to the council, a move Iran would perceive as a threat.

The three countries offered Iran incentives to give up suspicious aspects of its nuclear program. A deal was reached but fell apart in June after the IAEA reported that Iran wasn't fully cooperating with its inspectors.

Iran responded to the report and an angry rebuke by the IAEA's board by restarting the work it had suspended, including the construction of centrifuge equipment that could be used to enrich uranium.

The reaction was unexpected and hurt relations between Europe and Iran. Analysts said the tone of the next report will affect the possibility of talks between the two sides.

"If the IAEA says Iran is cooperating, then there is still an opening for the Europeans and others to work with them," said Shireen T. Hunter, an Iran specialist at the Center for Strategic and International Studies. "But if the report is critical and comes up with examples of poor cooperation or secret work, then obviously pressure will mount to send this to the United Nations."

Flynt Leverett, a visiting fellow at the Brookings Institution's Saban Center and a former staff member of the National Security Council in the Bush administration, said he doubts that Iran will be referred to the United Nations at this point.

"The administration's Iran policy right now on the nuclear issue is to get the Europeans fired up enough to go the council," Leverett said. "But short of a sighting of a mushroom cloud, I don't think there is anything in this report that can get the board to vote for a Security Council referral. I just don't think that's where the Europeans or the other people on the board are right now."

The IAEA is investigating Iran's links to Pakistani Abdul Qadeer Khan, who once ran a nuclear black market.