The White House says the detention in Baghdad of several Iranians suspected of inciting attacks against Iraqi troops validates the U.S. claim of Iranian meddling in Iraq.
A spokesman said Monday U.S. officials want to finish their investigation of the detained Iranians before characterizing their activities.
He said two detainees with diplomatic immunity were handed over to Iraqi authorities, and that U.S. officials are working with Iraq's government on the status of the remaining ones.
U.S. forces in Baghdad detained several Iranians in raids last week.
Iraqi officials say President Jalal Talabani had invited two of the Iranians to the capital, and that he was unhappy American forces had detained them.
The New York Times reported that at least four Iranians remain in U.S. custody, and that the U.S. says they are senior Iranian military officials.
The United States has accused Iranian agents of stirring up sectarian violence in Iraq by arming and training Shi'ite militias. Tehran says it only has political and religious links with Iraqi Shi'ites.
In other news, British troops killed seven gunmen and demolished an Iraqi police station in the southern city of Basra early Monday to stop rogue officers from using the site to commit crimes.
In Baghdad, Iraqi police say a car bomb exploded in a mainly Shi'ite neighborhood, killing at least 10 people.
Some information for this report provided by AP, AFP and Reuters.