Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, said the United States would continue to confront Iraq militarily inside Iraq and exert diplomatic and economic pressure to influence its behavior.
But in a news conference here, Mullen said Iran had not responded to such pressure in the past and instead appears to have steadily increased its support for militant groups in Iraq and elsewhere over the past two years.
"I have no expectations that we are going to get into a conflict with Iran in the immediate future," he said.
"But I am concerned over time that in these last couple of years these tensions continue to rise, Iran does not respond and in fact seems to be ratcheting it up in terms of their support for terrorism.
"And I am concerned about where that goes in the long term. And I don't have a time and I don't have a solution with respect to that. I think we need to continue to press using all available means," he said.
Mullen's comments carry particular weight because he is seen as a moderate who has voiced concerns about the impact of the war in Iraq on US military forces.
He said Friday that a third conflict in the Middle East would be "extremely stressing," but added that it would be "a mistake to think we're out of combat power."
"I'm extremely concerned about what I believe is an increasingly lethal and malign influence by that government, and the Quds Force in particular, in Iraq and throughout the Middle East.
He said the Iraqi campaign to crackdown on armed Shiite militants in the southern city of Basra "reveal just how much and just how far to foment instability."
Some Iranian-made weapons found during the operation had date stamps on them that were "very recent," he said.
He said the leader of the Quds Force, a covert paramilitary arm of the Iranian Revolutionary Guards, was involved in the Basra violence. But Mullen said he had seen no "smoking gun" proof that the Iranian leadership was aware of it.
"The Iranian government pledged to halt such activities just some months ago. It's plainly obvious they have not. Indeed they seem to have gone the other way," he said.
"Our focus militarily is on dealing with this threat insde Iraq, and while all options certainly remain open, I am convinced the solution right now still lies in using other levers of national power, including diplomatic, financial and international pressure," he said.