"They are currently putting people into Iraq to do things that are harmful to the future of Iraq," Donald Rumsfeld told a Pentagon news conference on Tuesday.
"And it is something that they, I think, will look back on as having been an error in judgment."
Rumsfeld offered few details about his allegation of interference by Iran, which fought an eight-year war with Saddam Hussein's Iraq in the 1980s and shares a largely unguarded border.
He did not elaborate except to say the infiltrators were members of the Al Quds Division of Iran's Revolutionary Guards, the network of soldiers and vigilantes responsible for operations outside Iranian territory and whose mandate is to defeat threats to the 1979 Islamic revolution.
Rumsfeld and other US officials have complained previously of Iranian complicity in the movement of explosives and bomb-making material across the border into Iraq, but Rumsfeld had not mentioned Iranian forces before.
He initially said the infiltrators were doing "things that are harmful to the future of Iraq".
When asked specifically whether they were gathering intelligence or fomenting violence, he said he did not know what their mission was.
"The most recent reports have to do with individuals crossing the border"
Marine General Peter Pace, Chairman,
Joint Chiefs of Staff
Appearing with Rumsfeld, General Peter Pace, chairman of the joint chiefs of staff, said although there have been indications of Iranian-manufactured weapons going into Iraq, "the most recent reports have to do with individuals crossing the border".
He said he had an estimate of the number but would not reveal it.
Pace said he did not know whether the Iranians were sent by their government.
Asked the same question, Rumsfeld replied: "Of course. Quds force, the Revolutionary Guard, doesn't go milling around willy-nilly, one would think."
Violence in Iraq
Rumsfeld rejected the idea that Iraq had slipped into civil war, asserting that media reports had overstated recent violence there.
"I do not believe they are in a civil war today," Rumsfeld said. However, he added: "There has always been a potential for civil war."
The secretary spoke almost two weeks after the 22 February bombing of a Shia shrine in Samarra, which was followed by the deaths of hundreds of Iraqis.
Hoping to keep Iraqi efforts to form a unity government moving forwards, US officials have acknowledged unease about the violence but have repeatedly denied that they fear a full-scale civil war was erupting.
"There has always been a potential for civil war"
US Defence Secretary
Rumsfeld acknowledged the attack on the mosque had delayed efforts to form a government in which Shia, Sunnis and Kurds would share power.
"Their efforts to fashion a unity government that will represent all elements of their society is clearly being delayed by the situation in Iraq," he said.
But Rumsfeld also asserted that Iraqi leaders had thus far passed the test of holding the country together and containing efforts to ignite a civil war.
"They have to be fully aware that if this does not work, they and all of the people who have supported them lose everything, if this turns into a civil war. They can't want that.
"My impression is they will sort through this and fashion a government of some sort."