In a report sure to further complicate the U.S.-led drive for additional sanctions against Iran, the atomic watchdog agency of the United Nations has given Iran relatively good marks for its recent cooperation in clearing up suspicions over past nuclear work.

"Iran has provided sufficient access to individuals and has responded in a timely manner to questions and provided clarifications and amplifications on issues raised," the International Atomic Energy Agency said.

The report was acquired by a Washington think tank, the Institute for Science and International Security, and released Thursday on its website.

The IAEA said it had been "able to verify the non-diversion of declared nuclear material in Iran." However, the agency warned that its understanding of Iran's current nuclear operations "is diminishing" because of Tehran's earlier decision not to follow a separate IAEA protocol that would permit more extensive inspections. The Vienna-based agency also termed Iran's cooperation "reactive rather than proactive" and said the probe of Tehran's once secret nuclear history would continue. Also worrisome: Media reports suggest that Tehran recently denied IAEA requests to interview at least two Iranians about their nuclear work. That raises concern within the Bush administration that IAEA chief Mohamed ElBaradei, who has suggested that it is unrealistic to bar Iran from uranium enrichment, is too eager to put a positive spin on Iran's cooperation.

In particular, agency investigators are still waiting for Iranian explanations and documents about uranium contamination at a technical university, the operations of a uranium mine, and alleged studies related to atomic weapons research, including high-explosive testing and the design of a missile re-entry vehicle. Just last week, the IAEA received from Iranian officials a copy of a 15-page document laying out how to reduce uranium hexafluoride gas into uranium metal and cast it into hemispheres—in the fashion of a nuclear bomb. Why Iran had not shared the document earlier received no explicit comment.

The Bush administration and most of its European allies say the report will not prevent them from seeking a third, tougher round of sanctions at the U.N. Security Council.

The British Foreign Office called on Tehran to "come clean" on unresolved issues "without delay." White House spokeswoman Dana Perino said, "We believe that collective cooperation is not good enough .... And unfortunately, this report makes clear that Iran seems uninterested in working with the rest of the world."

But Russia and China, which have resisted more sanctions and cited the new Iranian-IAEA cooperation as a reason to show patience, are likely to seize on the report to buttress their stand.

Skeptics believe that Iran has released documents bit by bit as part of an effort to delay more international penalties—and to split Russia and China from the West, if possible. Iran said the report shows that new sanctions would be illegal and that the accusations against it are "baseless."

The report was unequivocal in verifying that despite Security Council demands to the contrary, Iran is proceeding with uranium enrichment and construction of a heavy-water production plant, which would supply a heavy-water reactor capable of producing plutonium. The IAEA found that Iran had installed nearly 3,000 centrifuges at its main uranium enrichment plant as of November 3.

A group of six countries is seeking talks with Iran—but only if it first suspends work on making nuclear fuel. Iran refuses. British Prime Minister Gordon Brown said earlier this week that without a "positive outcome" from the IAEA report and another one from the European Union, he would advocate harsher sanctions, including hitting Iran's oil, gas, and financial sectors.