US ambassador Jackie Sanders told the International Atomic Energy Agency that Iran has continued to deny IAEA inspectors "the transparency and cooperation they need to perform their duties" and that Tehran was "cynically" manipulating "the nuclear nonproliferation regime in the pursuit of nuclear weapons."

Sanders, who is based in Geneva but heads the US delegation to the IAEA's 35-nation board of governors meeting in Vienna this week, said "there remain an alarming number of unresolved questions about Iran's nuclear program."

Among them are why the Islamic Republic is building a heavy-water reactor that can make weapons-grade plutonium and why Iran was late in reporting on construction of "deep tunnels for storage of nuclear material" at a site that carries out the first stages of uranium enrichment.

Enrichment uses centrifuges to refine out what can be reactor fuel but also the explosive core of atom bombs.

Sanders said the IAEA cannot put off "forever" bringing Iran before the Security Council, something the United States has been seeking for almost two years as it says Tehran is in clear violation of the nuclear Non-Proliferation Treaty (NPT).

"The board cannot ignore forever its statutory responsibility to report this matter" to the Council, Sanders said, according to a text of her comments made available to the press.

The Security Council would have "the authority to require that Iran take all necessary corrective measures, including those steps called for by the (IAEA) board that Iran has failed to take," Sanders said.

She said these included "the authority to require and enforce a suspension of Iran's enrichment-related and reprocessing activities."

Britain, France and Germany, which agreed with Iran on the enrichment suspension and want Tehran to make this permanent in return for trade and security benefits, joined in the US concern over Iranian failures to report fully.

The European trio said in a statement to the board that "Iran has carried out operations of cleaning and quality control on certain centrifuge components, which has caused us serious concern."

The trio said they understood the suspension "as a voluntary commitment to suspend all, meaning each and every, enrichment related activity, without exception. We urge Iran to keep to this voluntary commitment."

Iran says its suspension is temporary since the NPT gives it the right to exploit the nuclear fuel cycle for peaceful purposes and that it has corrected all reporting failures.

Sanders said IAEA chief Mohamed ElBaradei should report to the agency's board ahead of its next meeting in June, after not having reported to the current meeting, as this would clear the way for action against Iran if necessary.

Iranian delegation chief Cyrus Nasseri told reporters that the United States wanted to get the issue before the Security Council because it "might be in the driver's seat there" while Washington was isolated at the IAEA.

ElBaradei meanwhile also told reporters it was now up to Tehran to "come clean" on nuclear issues by allowing wider access to IAEA inspectors.

The IAEA has failed in two years of investigation to reach a conclusion as to whether Iran is pursuing a nuclear weapons program.

IAEA deputy director Pierre Goldschmidt had Tuesday outlined to the board key areas where Iran is refusing to cooperate with UN inspectors.

These include blocking a follow-up visit to the Parchin military facility where Washington charges Tehran is simulating testing of nuclear weapons.

Nasseri said Iran was not allowing a second visit to Parchin, after a first one in January, in part because it was concerned about information leaks "in view of potential threats of military strikes against safeguarded and other facilities visited by the agency in Iran," in a clear reference to the United States.

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