Two of those detained were visiting as guests of Iraqi President Jalal Talabani, his spokesman said.
The White House confirmed the detentions.
"We suspect this event validates our claims about Iranian meddling, but we want to finish our investigation of the detained Iranians before characterizing their activities," White House spokesman Alex Conant said Monday. "We will be better able to explain what this means about the larger picture after we finish our investigation."
He said that a routine raid on suspected insurgents netted the Iranians. Two had diplomatic immunity and were released to the Iraqi government (GOI), which then released them to Iran, Conant said.
"We continue to work with the GOI on the status of the remaining detainees," Conant said. "That investigation is going well."
The incident comes at a delicate diplomatic time. The Iranian president, Mahmoud Ahmadinejad is trying to expand Iran's role in Iraq as a counter to U.S. influence in the Gulf region. The Bush administration, meanwhile, has resisted pressure for a diplomatic push that would involve all of Iraq's neighbors — including Iran and Syria.
The New York Times reported Monday that U.S. forces were holding four Iranians, including some seized at the compound of Abdul-Aziz al-Hakim, the leader of the largest Shiite bloc in Iraq's parliament, who met with President Bush earlier this month at the White House. The Times said U.S. forces also stopped an Iranian embassy car last week and detained two Iranian diplomats, their Iranian guards and an Iraqi driver. The diplomats were later released by Iraq, it said.
Hakim is not only one of Iraq's leading Shiites, reports CBS News chief White House correspondent Jim Axelrod, he is often thought of as an alternative to Iraqi Prime Minister Nouri al-Malikial if al-Maliki proves unable to control the violence. In fact, Hakim was at the White House three weeks ago and that's one of the reasons why.
"Two Iranians who are in Iraq at the invitation of the president have been apprehended by the Americans," said Hiwa Osman, Talabani's media adviser. "The president is unhappy about it."
Osman had no further details. At the White House, Conant declined to characterize the Iranians' activities until the investigation is concluded. "
"We continue to work with the GOI (Government of Iraq) on the status of the remaining detainees," Conant said. "That investigation is going well."
A Pentagon official, speaking on condition of anonymity because of the delicacy of the situation, confirmed that Iranians had been among those detained during recent operations against people planning attacks against coalition forces and Iraqis.
The United States has accused Iran of supplying money, weapons components and training to Shiite militia in Iraq, as well as technology for roadside bombs, the biggest killer of American forces in Iraq. Iran says it only has political and religious links with Iraqi Shiites.
Late last month, Talabani visited Iran for two days of talks with government officials to seek their support in quelling the raging sectarian violence in Iraq. Iran, a Shiite Muslim country, has considerable influence among Iraq's Shiite majority — elements of which have been blamed for the bulk of the recent attacks.
Talabani is a member of Iraq's Kurdish minority, but he had close ties with Iranian officials before Saddam Hussein was overthrown by the U.S.-led invasion in 2003.