The state TV reported that the scientist, Shahram Amiri, entered the embassy's office representing Iranian interests in the U.S. and demanded an "immediate return" to Iran.
Mostafa Rahmani, head of the Iranian office in Washington, said the TV report was true but would not elaborate. He said Iran's foreign ministry would "release details later."
Pakistan's Foreign Ministry spokesman Abdul Basit said that according to Rahmani, the scientist showed up around 6:30 p.m. Monday at the Iranian interest section office and has been with them since.
"We understand from Dr. Rahmani that they are making arrangements for his repatriation to Iran," Basit told The Associated Press in Islamabad. He did not know when a transfer would occur or whether Pakistan would have a hand in making the travel arrangements.
Amiri disappeared while on a pilgrimage to Saudi Arabia in June 2009. Iran has repeatedly claimed that the U.S. abducted Amiri - charges the U.S. denies.
Before he disappeared, Amiri worked at Tehran's Malek Ashtar University, an institution closely connected to the country's powerful Revolutionary Guard.
U.S. media reported in March that Amiri defected to the U.S. and was assisting the CIA in efforts to undermine Iran's nuclear program.
Iran's state TV has periodically showed purported videos of Amiri claiming abduction and torture by the U.S.
One of the videos, aired in early June, showed a man wearing headphones and speaking through what appeared to be a webcam, saying he was abducted while on a pilgrimage to Medina, injected with a tranquilizer and taken to the United States.
Iran has previously hinted it would trade three American prisoners it has held since last July for a number of Iranians allegedly detained by the United States, including Amiri. An Iranian spokesman later said no such deal was in the works.
The United Nations in early June slapped a fourth round of sanctions on Tehran over its refusal to curtain its nuclear program. The West accuses Iran of seeking to develop atomic weapons, a charge that Iran denies. Iran maintains that its nuclear research is for peaceful purposes.
The U.S. and Iran have had no diplomatic relations following the 1979 Islamic revolution. The Pakistan embassy in Washington looks after Iranian interests in the U.S.
---
Associated Press Writer Nahal Toosi in Islamabad contributed to this report.
(This version CORRECTS that Iranian scientist showed up at Washington office around 6:30 p.m. and not 6:30 a.m.)
