Manouchehr Mottaki accused the US of abducting the scientist in Saudi Arabia
Iran has accused the US of abducting one of its nuclear scientists who has been missing since June.
Shahram Amiri disappeared in Saudi Arabia while on a Muslim pilgrimage.
Foreign Minister Manouchehr Mottaki told journalists the Iranians expected the US to return Mr Amiri, and asked the Saudis to co-operate too.
Another Iranian official admitted on Tuesday that Mr Amiri was a nuclear scientist, something Tehran had not confirmed in previous announcements.
"Based on existing pieces of evidence that we have at our disposal the Americans had a role in Mr Amiri's abduction," Mr Mottaki said.
"The Americans did abduct him. Therefore we expect the American government to return him."
Mr Amiri worked as a researcher at Tehran's Malek Ashtar University, according to Iran's state-run Press TV channel.
However, some reports said he had also been employed by Iran's Atomic Energy Organisation, and had wanted to seek asylum abroad.
Iranian officials had refused to confirm this until now.
Mr Amiri was a nuclear scientist, news agency Mehr quoted a foreign ministry spokesman as confirming.
"Riyadh has handed over Iran's nuclear scientist Amiri to America," Ramin Mehmanparast said.
Washington has denied any knowledge of the scientist.
The US and its Western allies suspect Iran of secretly developing nuclear weapons - a claim denied by Tehran.