Three Iranian diplomats were detained and interrogated by U.S. forces in Iraq on Friday.
Mohammad Ali Hosseini, the spokesman, condemned the detentions, which he said occurred while they were traveling from Iraq to Iran on Friday to spend the weekend back home.
The three were first detained by Iraqi police, then later held by U.S. troops and questioned for several hours, Hosseini said, stressing that Iraqi authorities had been informed in advance of the diplomats' travel plans.
The diplomats were not identified by name. They were later handed back to Iraqi police who took them to the Iranian embassy in Baghdad, where they are at present, Hosseini said.
He warned that the treatment of the Iranian diplomats was contrary to international relations and amounted to an interference in Iraqi internal affairs by the United States.
Detentions of Iranian nationals have led to increased tensions between Washington and Tehran since U.S. troops in January arrested five Iranians in the northern Iraqi city of Irbil.
The U.S. military has said the five, who remain in U.S. custody, are suspected of links to a network supplying arms to Iraqi insurgents - an accusation denied by Iran which maintains they are diplomats.
Iran, meanwhile, holds in jail four Iranian-American scholars and activists charged with acting against Iranian national security.
U.S. officials in Baghdad did not immediately comment the claim of the Friday detentions.
US President Joe Biden and Iraqi Prime Minister Mustafa Al Qadimi signed an agreement on July 26 to formally end the USA's military presence in the country by the end of the year