"No one said this would be easy or quick, and no one should be lulled into a false sense of security by accurate air strikes," Pentagon spokesman Rear Admiral John Kirby told reporters.
"We will not, we cannot bomb them into obscurity."
Kirby spoke a week after Washington launched air raids against the IS group in Syria, with dozens of bombing runs flown by American and Arab aircraft.
The raids have struck IS-controlled oil refineries, tanks, artillery, buildings and other targets, even as the militants continued to gain ground in some areas, including near a contested town near the Turkish border where Kurdish forces have been pushed back.
Kirby criticised some media coverage as raising unrealistic expectations about the air campaign in Syria and Iraq.
Commanders from the outset had made clear that air power alone would not be enough while a long-term effort would be needed to train and arm Syrian rebel forces and strengthen Iraq's army, he said.
"Even as we share the sense of urgency about this group, we must also share a sense of strategic patience about this entire effort. And I think some of that has been lacking," Kirby said.
IS fighters are no longer moving openly in large groups and are "dispersing" to avoid being hit from the air, he added.
But he acknowledged the group still posed a threat and in some cases had seized more territory -- even as it faces bombing from the air.
Effective air raids did not mean that "they aren't still trying and in some cases succeeding at taking and holding ground," Kirby said.
"We've been pretty honest about the fact that military action alone will not win this effort, but that shouldn't be taken as an admission of ineffectiveness.
"And one of the ways we know we're having an effect is precisely because the terrorists have had to change their tactics and their communications and their command and control," due to the air raids, he said.