Mystery surrounds the fate of a missing Iranian scientist who is reported to have been involved in Tehran's controversial nuclear programme.
Shahram Amiri disappeared in June in Saudi Arabia while on a pilgrimage.
Iran on Wednesday accused the US of involvement in his disappearance, but did not confirm media reports that Mr Amiri was indeed a nuclear scientist.
In response, a US state department spokesman said Washington had no information about the case.
"We have found documents that prove US interference in the disappearance of the Iranian pilgrim Shahram Amiri in Saudi Arabia," Iranian Foreign Minister Manouchehr Mottaki was quoted as saying by Iran's Fars news agency.
"We hold Saudi Arabia responsible for Shahram Amiri's situation and consider the US to be involved in his arrest," the minister added.
In Washington, state department spokesman Ian Kelly said only that "the case is not familiar to us".
There have been no public comments from Saudi Arabia.
Mr Amiri worked as a researcher at Tehran's Malek Ashtar University, according to Iran's state-run Press TV channel.
However, some reports in Iran's media say he was also an employee of Iran's Atomic Energy Organisation and wanted to seek asylum abroad.
The US and its Western allies suspect Iran of secretly developing nuclear weapons - a claim denied by Tehran.