General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, said there was no intention now to place American military advisers on the ground in direct combat. US assistance is taking other forms, including air strikes.
Still, Dempsey outlined scenarios in which he might recommend having US troops do more, potentially accompanying Iraqis during complicated offensives, such as a battle to retake the northern city of Mosul from Islamic State fighters.
"It could very well be part of that particular mission - to provide close combat advising or accompanying for that mission," Dempsey said.
Dempsey acknowledged that Obama's "stated policy is that we will not have US ground forces in direct combat."
"But he has told me as well to come back to him on a case-by-case basis," he said.
Obama said last week he would lead an alliance to defeat Islamic State militants in Iraq and Syria, plunging the United States into a conflict in which nearly every country in the Middle East has a stake.
But Obama also ruled out a combat mission, saying "we will not get dragged into another ground war in Iraq." How exactly America's role might evolve in the open-ended conflict remains unclear, however.
Responding to Dempsey's comments, the White House said Obama’s military advisers had to plan for many possibilities and that overall policy had not changed - that Obama would not deploy US troops in a combat role in Iraq or Syria.
White House spokesman Josh Earnest told reporters that Dempsey was "referring to a hypothetical scenario in which there might be a future situation where he might make a tactical recommendation to the president as it relates to ground troops."
Dempsey's spokesman also issued a statement stressing that the four-star general's exchange in the Senate was not about "employing U.S. ground combat units in Iraq."
Dempsey was testifying before the Senate Armed Services Committee, along with US Defense Secretary Chuck Hagel, as the Obama administration makes its case to Congress for broadening operations against the Sunni militants, which would include US air strikes in Syria for the first time.
The US military's Central Command is due to brief Obama on its plans on Wednesday. Hagel said those plans envision striking the militant group's safe havens in Syria to knock out infrastructure, logistics and command capabilities.
Dempsey said the strikes would degrade the group's capabilities as broader efforts get under way, including training of more than 5,000 Syrian rebels.
"This won't look like a 'shock and awe' campaign because that's simply not how (the Islamic State militants' group) is organized. But it will be a persistent and sustainable campaign," Dempsey told the Senate Armed Services Committee.
"Shock and awe" was a term popularly used to describe the initial air assault on Baghdad in the US campaign to oust Saddam Hussein in 2003, and refers to use of overwhelming force to undermine an enemy's will to fight.
Congress is expected to approve this week a request from Obama for authorization to arm and train moderate Syrian rebels, one part of his program.
Still, Hagel acknowledged the number of Syrian fighters that could be trained over the course of the year would only put the opposition on a path to roll back Islamic State fighters.
"Five thousand alone is not going to be able to turn the tide. We recognize that," Hagel said.
The Senate hearing was repeatedly interrupted by anti-war protesters, shouting slogans such as, "There is no military solution." One protester was escorted out of the room while holding a sign that read: "More war = More extremism."
Senator Angus King of Maine, expressing concern that the United States would be drawn into interminable fights against extremist groups around the world from Iraq to Syria to Africa, said: "This is geopolitical Wack-a-mole."