Raising a new complaint about Iran, Defense Secretary Donald H. Rumsfeld accused Tehran of dispatching elements of its Revolutionary Guard to stir trouble inside Iraq.
At the same time, he rejected the idea that Iraq has slipped into civil war, asserting that media reports have overstated recent violence there.
Rumsfeld offered few details about his allegation of interference by Iran, which fought an eight-year-long 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 whose mandate is to defeat threats to the 1979 Islamic revolution. The Al Quds Division is responsible for operations outside Iranian territory.
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.