"There is solid evidence that Iranian agents are involved in these networks and that they are working with individuals and groups in Iraq and that they are being sent there by the Iranian government," said State Department spokesman Sean McCormack.
"And I would expect that … in the near future, we are going to try to talk a little bit more in public — to the extent that we can because, again, you’re dealing with intelligence information — about what we know of Iranian support for these networks," he added.
The United States, which accuses Iran of funding and equipping Shiite militias in Iraq, arrested five Iranians at an office in the northern Iraqi city of Arbil on January 11, accusing them of being agents for Tehran, arming militias and inciting anti-US attacks in Iraq.
The arrests triggered a diplomatic row, with Tehran accusing US forces in Iraq of violating international diplomatic regulations, but Washington and the US military in Iraq maintain that those arrested had no diplomatic status.
Iraqi Foreign Minister Hoshyar Zebari has said that the five Iranians had been working in Arbil with official sanction, but that their "liaison office" had not yet become a full consulate.
McCormack rejected the idea the detainees were working at a "liaison office."
"One thing we can tell you is they’re not diplomats," he said, adding that the detainees were "still in the custody of multinational forces."
The spokesman refused to say whether the US had evidence linking Iran to any of the explosives or bombs that have been set off in Iraq.
However, he said his government was sure of it.
"You don’t necessarily have to construct something in Iran in order for it to be a threat to the US or British troops from the Iranian regime," he said.
"There are a lot of different ways you can do that. You can bring the know-how. You can train other people in Iraq to do that. So there are a lot of different ways to do it.
"I would suspect that they’re probably trying to hide their tracks somewhat, so you’re not going to have a "made in Iran" stamp on all of these items. But certainly the technology and the know-how originates in Iran," said McCormack.