Iranian nuclear scientist Shamram Amiri turned up at Pakistan's embassy in Washington on Monday and has decided to return to Iran, a U.S. official said on Tuesday.
"Mr. Amiri has been in the United States of his own free will and has decided to return to Iran of his own free will," the U.S. official said, speaking on condition of anonymity.
Amiri is awaiting travel documents from a third country through which he plans to travel on his way back to Iran, according to the official.
Iran's state radio said earlier on Tuesday that Amiri, a university researcher who disappeared during a pilgrimage to Saudi Arabia a year ago, was at the mission. "He wants to be returned to Iran immediately," state radio said in its report.
A Pakistani foreign ministry official in Islamabad also confirmed to Reuters that the scientist was in the Iranian section of Pakistan's embassy, not in the embassy itself, Reuters said.
Iran and the United States severed diplomatic ties shortly after the country's 1979 Islamic revolution. The Pakistani embassy looks after Iran's interests in the United States.
Iran's semi-official Fars news agency said Amiri was handed over to the embassy by U.S. agents, calling it a defeat for "America's intelligence services".
"Because of Iran's media and intelligence activities, the American government had to back down and hand over Amiri to the embassy on Monday night," Fars said.
Officials at the State Department and at the Pakistan embassy in Washington were not available for comment.
Iran accuses the United States and Saudi Arabia of abducting Amiri, who worked for Iran's Atomic Energy Organisation. U.S. and Saudi officials have denied the accusation.
Iran has summoned at least twice the Swiss ambassador in Tehran and handed over documents which it said showed Amiri had been kidnapped by the United States. U.S. interests in Tehran are handled by the Swiss embassy.
Tehran also demanded the release and repatriation of Amiri along with 10 other Iranian nationals who it says have been "illegally detained" in the United States.
"We expect that based on the US administration's obligations... the US authorities will announce the results of their investigation regarding this Iranian national," foreign ministry spokesman Ramin Mehmanaparast had said.
On June 29, Iranian television screened a video of a man claiming to be Amiri and saying that he had managed to escape from the hands of US intelligence agents in Virginia.
In one video, a man identified as Amiri, said he was taken to the United States and tortured.
"I could be re-arrested at any time by US agents... I am not free and I'm not allowed to contact my family. If something happens and I do not return home alive, the US government will be responsible," he said.
"I ask Iranian officials and organisations that defend human rights to raise pressure on the US government for my release and return to my country," the man said, adding he has not "betrayed" Iran.
Iranian authorities have repeatedly accused the United States of kidnapping and illegally detaining Iranians, including a former deputy defence minister who disappeared in 2007.
In another video on the Internet, a man also said to be the scientist said he was studying in the United States.
In a third video, a man describing himself as Amiri said he had fled from U.S. "agents" and was in hiding, urging human rights groups to help him to return to Iran.
In March, ABC news said Amiri had defected to the United States and was helping the CIA. Before Amiri disappeared, he also worked at Iran's Malek Ashtar University, an institution closely connected to the country's elite Revolutionary Guards.
Tehran initially refused to acknowledge Amiri's involvement in Iran's nuclear programme, which the West claims is being used to develop nuclear weapons and which Iran says is designed to generate electricity.