An Iranian nuclear scientist who vanished more than a year ago mysteriously turned up in Washington Tuesday claiming to have been kidnapped, but the United States denied that he was held against his will.

Iran, which is locked in a standoff with the West over its suspected pursuit of nuclear weapons, has repeatedly accused the U.S. Central Intelligence Agency of abducting Shahram Amiri, who worked for Iran's Atomic Energy Organization.

Amiri, who went missing during a pilgrimage to Saudi Arabia more than a year ago, appeared at the Iranian interests section of the Pakistani embassy, which represents Iran because Tehran and Washington have no diplomatic relations.

A man claiming to be Amiri has variously claimed in recent videos that he was kidnapped and tortured; that he was studying in the United States and that he had fled U.S. "agents" and wanted human rights groups to help him return to Iran.

Amiri was quoted by Iranian state TV Tuesday as saying "my kidnapping was a disgraceful act for America." His comment fuelled speculation that he may have had valuable intelligence about the Iranian nuclear program.

In March, ABC News reported he defected and was helping the CIA. U.S. officials Tuesday said Amiri had decided to return to Iran of his own volition.

The United States accuses Iran of using its civil nuclear program as a cover to develop weapons. Iran, which has been hit by four rounds of UN Security Council sanctions over the nuclear issue, says its program is to generate power.

Intelligence about the program is at a premium for the U.S., which fears that an Iranian nuclear weapon could threaten its close ally, Israel, as well as oil supplies from the Gulf and friendly nations in Europe.