BBC News
Shahram Amiri disappeared a year ago while on pilgrimage in Saudi Arabia.
Earlier this month, Tehran said it had proof he was being held in the US. The US denies having abducted him.
The allegation came after three videos purportedly of Mr Amiri containing contradictory information as to his whereabouts emerged.
The first said he had been kidnapped, the second that he was living freely in Arizona, and the third that he had escaped from his captors.
US denial
“A few hours ago Shahram Amiri took refuge at Iran’s interest section at the Pakistan embassy in Washington, wanting to return to Iran immediately,” Iranian state radio reported on Tuesday morning.
The US cut diplomatic relations with Iran shortly after the 1979 Islamic revolution and the Pakistan embassy in Washington looks after Iran’s interests.
The US has strenuously denied abducting Mr Amiri, while ABC News reported in March that he had defected and was helping the CIA.
Iranian media say Mr Amiri worked as a researcher at a university in Tehran, but some reports say he worked for the country’s atomic energy organisation and had in-depth knowledge of its controversial nuclear programme.
The first two videos, telling starkly contradictory stories, were posted on the video-sharing site YouTube on 8 June.
In the first, initially broadcast by Iranian television, a man purporting to be Mr Amiri says he was kidnapped by the US while on pilgrimage in the Saudi Arabian city of Medina and that he is now living in the US state of Arizona.
At the time the Iranian government described the video as evidence that he was being held in the US against his will.
Call for assistance
In the second, posted hours later on YouTube, a similar-looking man claiming to be the scientist says he is happy in the US, living in freedom and safety.
In the third video, which was broadcast by Iranian state TV on 29 June, a man claiming to be the missing scientist says: “I, Shahram Amiri, am a national of the Islamic Republic of Iran and a few minutes ago I succeeded in escaping US security agents in Virginia.
“Presently, I am producing this video in a safe place. I could be re-arrested at any time.”
The man in the video also dismisses the second video, in which it was claimed that the scientist was living freely in the US, as “a complete fabrication”.
“I am not free here and I am not permitted to contact my family. If something happens and I do not return home alive, the US government will be responsible.”
The video finishes with the man urging Iranian officials and human rights organisations to “put pressure on the US government for my release and return”.
“I was not prepared to betray my country under any kind of threats or bribery by the US government,” he adds.
A US official told the AFP news agency at the time that the allegations were “ludicrous”.
