Controversy over a possible missed U.S. opportunity for rapprochement with Iran grew on Wednesday as former aide accused Secretary of State Condoleezza Rice of misleading Congress on the issue.

Flynt Leverett, who worked on the National Security Council when it was headed by Rice, said a proposal vetted by Tehran's most senior leaders was sent to the United States in May 2003 and was akin to the 1972 U.S. opening to China.

Speaking at a conference on Capitol Hill, Leverett said he was confident it was seen by Rice and then-Secretary of State Colin Powell but "the administration rejected the overture."

Rice's spokesman denied she misled Congress and reiterated that she did not see the proposal.

Separately, Undersecretary of State Nicholas Burns warned Iran it risked further U.N. and other sanctions if it did not halt uranium enrichment as the U.N. Security Council demanded.

He stressed there was still time for diplomacy before Iran reached a critical point in its nuclear capability and said conflict with Iran was not inevitable.

Washington remains patient and committed to negotiations with Tehran and its carrot-and-stick approach with other major powers is influencing Iran's internal debate, Burns told the Brookings Institution think tank.

Leverett, speaking at a conference hosted by the New America Foundation think tank, said the 2003 overture "was a serious proposal" for a comprehensive agenda for U.S.-Iranian rapprochement.

"The Bush administration up to and including Secretary Rice is misleading Congress and the American public about the Iran proposal," he said.

Testifying before a U.S. Congress committee last week, Rice, said about Leverett's previous public comments on the Iranian proposal: "I don't know what Flynt Leverett's talking about."

She faulted him for not telling her, "We have a proposal from Iran and we really ought to take it."

On Wednesday, State Department spokesman Sean McCormack said: "What she said is she has no recollection of having seen it. She has said that repeatedly."

Leverett and others have represented the proposal as a missed opportunity that could have defused tensions with Iran which have grown to the point that the U.S. administration has been forced to deny it plans military action against Tehran.

Leverett said Rice should apologize for calling his competence into question.

He said he had left the National Security Council, which advises the president on security issues, in March 2003 before the Iranian proposal was received. He returned to the CIA where he previously worked and soon after left government. Hence, he was not in a position to make this case directly to Rice, he said.

Leverett said Powell, in a conversation about the Iranian proposal, told him he "couldn't sell it at the White House." This was evidence it had been discussed there, he said.

The proposal was transmitted in May 2003 by the Swiss ambassador in Tehran, Tim Guldimann, who represented U.S. interests there. Washington has not had diplomatic relations with Iran since two years after the 1979 Islamic revolution.

According to a copy of the proposal posted on The Washington Post Web site and cited by Leverett, it contains considerable detail about approaching issues of central interest to the United States and Iran.

This included an end to Iran's support for anti-Israel militants and acceptance of Israel's right to exist.

It carried a cover letter from Guldimann, who said the proposal was approved by Iran's supreme religious leader, Ayatollah Ali Khameini, and then-President Mohammed Khatami.