Iran has formally protested to the US over the arrest in Iraq of two Iranians that Tehran said were diplomats invited there by the Iraqi government.
The US said it was investigating several Iranians detained in Iraq
American officials said US forces had detained several Iranians suspected of planning attacks in Iraq.
A spokesman for the Iraqi President, Jalal Talabani, said he is unhappy about the US action.
The White House said the arrests validated US claims of Iranian "meddling" in Iraq.
The Swiss ambassador in Tehran, who looks after US interests in Iran, was called to the foreign office to discuss the arrests, the Iranian news agency, Irna ,said.
Iranian foreign ministry spokesman Mohammad Ali Hoseyni said the arrests contravened international law and might have "unpleasant repercussions," Irna reported.
In Baghdad, a spokesman for President Talabani said Iranian security personnel had been invited to Iraq by the president to help improve the situation.
"The president is unhappy about it, and has been making contact with the American authorities to arrange their release," Mr Talabani's spokesman, Hiwa Osman, told the BBC.
US officials said several Iranians had been arrested by US forces in Iraq, including two with diplomatic immunity, but said those two had been released.
White House spokesman Alex Conant said the US was investigating several Iranians still detained in Iraq.
"We suspect this event validates our claims about Iranian meddling, Mr Conant said.
"We will be better able to explain what this means about the larger picture after we finish our investigation."
The Iranians were arrested in two raids last week on suspicion of planning attacks in Iraq, the New York Times newspaper said.