Iran confirmed on Sunday that it had detained a fourth Iranian-American, who an Iranian news agency earlier said was being probed over security-related issues.
Iran previously said it charged two dual nationals with spying and said a third was being investigated for "anti-revolutionary" activities. But there was no word about the fourth, Ali Shakeri, until Friday's ISNA news agency report.
"Based on the news we have received, Ali Shakeri has been arrested," Foreign Ministry spokesman Mohammad Ali Hosseini told a weekly news conference.
He did not give details about any accusations against Shakeri, a Californian businessman, but ISNA said the probe involved "security-related charges". ISNA had not given a source for its report.
The Iranian judiciary said last month academic Haleh Esfandiari, social scientist Kian Tajbakhsh and journalist Parnaz Azima were accused of spying.
A judicial source later said Azima was detained for cooperating with "anti-revolutionary" media but freed on bail.
The United States has dismissed accusations against all four of those detained and demanded they be freed immediately.
Iran accuses the United States of backing a so-called "soft revolution", a perceived US plot to use intellectuals and others inside the country to undermine Iran's government.
Hosseini said Iran had complained to the Swiss embassy in Tehran, which handles US interests, about comments by US officials and media about a "soft revolution and for allocating some budgets in this field.
The United States, which cut ties with Tehran in 1980 over the seizing of US hostages by Iranian students, has a fund which Washington says is aimed at promoting democracy in the Islamic Republic and says it is to help those inside the country who want more civil liberties.
Some analysts have linked the detention of the dual nationals to a row with Washington over the arrest of five Iranians in Iraq by US forces in January.
Tehran insists the five are diplomats and has demanded their release. Washington says the men were backing militants in Iraq.
Washington and Tehran are also at loggerheads over Iran's nuclear program which Washington says is aimed at building atomic bombs -- a charge Tehran denies.
Despite the rows between the two arch-foes, US and Iranian officials held rare face-to-face talks in Baghdad last month to discuss security issues in Iraq.