The Iranian diplomat's fate, who was kidnapped by gunmen in Baghdad on Sunday night, remains unclear.
Iran has confirmed the kidnapping on Tuesday and said the gunmen "acted under US supervision," according to media reports on Wednesday.
Some 30 gunmen wearing Iraqi army uniforms in more than 10 military vehicles kidnapped Jalal Sharafi, the second secretary of the Iranian embassy, while he was driving through Masbah, near the Baghdad's Karradah neighbourhood.
Iran's Foreign Ministry spokesman Mohammad Ali Hosseini said the gunmen are "linked to the Iraqi Defense Ministry" and the kidnapping "was operated under the supervision of US forces there."
"Iran holds America responsible for the safety and life of the (abducted) diplomat," said Hosseini, "This aggressive action violates the international law and Iran strongly condemns it."
Iraqi Prime Minister Nouri al-Maliki had ordered to investigate the fate of the diplomat, added Hosseini.
However, the US authorities have denied any role in the matter. "We don't really know a whole lot about it at this point," White House spokesman Tony Snow said on Tuesday.
"We know that the Iraqi government is investigating."