US forces have stormed an Iranian consulate in the northern Iraqi town of Irbil and seized six members of staff.
The troops raided the building at about 0300 (0001GMT), taking away computers and papers, according to Kurdish media and senior local officials.
The US military would only confirm the detention of six people around Irbil.
The raid comes amid high Iran-US tension. The US accuses Iran of helping to fuel violence in Iraq and seeking nuclear arms. Iran denies both charges.
Tehran counters that US military involvement in the Middle East endangers the whole region.
A local TV station said Kurdish security forces had taken over the building after the Americans had left.
Irbil lies in Iraq's Kurdish-controlled north, about 350km (220 miles) from the capital Baghdad.
Reports say the Iranian consulate there was set up last year under an agreement with the Kurdish regional government to facilitate cross-border visits.
Iranian media said the country's embassy in Baghdad had sent a letter of protest about the raid to the Iraqi foreign ministry.
One Iranian news agency with a correspondent in Irbil says five US helicopters were used to land troops on the roof of the Iranian consulate.
It reports that a number of vehicles cordoned off the streets around the building, while US soldiers warned the occupants in three different languages that they should surrender or be killed.
In December, US troops detained a number of Iranians in Iraq, including two with diplomatic immunity who were later released.
Thursday's raid came as US President George W Bush unveiled his new strategy in Iraq, which included increasing troop numbers and a commitment to stop Iranian support for "our enemies in Iraq".