Kurdish peshmerga forces and Iraqi government troops have retaken 700 square kilometers (270 square miles) of ground mostly in northern Iraq, but the IS group still holds 55,000 square kilometers, spokesman Rear Admiral John Kirby told reporters.
That amounts to roughly one percent of IS-held territory changing hands since the US launched air raids in Iraq on August 8.
Kirby acknowledged that not much ground had been gained back so far but said that the air strikes had halted the momentum of the jihadists and bought time for the training of Baghdad government forces.
"I think we all recognize that it's a small percentage of the total right now. But we're only six, seven months into this thing, too," he said.
The US military has made clear the campaign against IS "is going to take time, to uproot these guys and to really get at them," he said.
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The Pentagon provided the figures after announcing recently that Iraqi and Kurdish troops had regained 700 square kilometers, without explaining what percentage that represented of all territory held by the IS militants.
By comparison, the Iraqi government held about 77,000 square kilometers and the Kurdish forces controlled roughly 56,000 square kilometers, he said.
Those numbers did not represent the entire territory of Iraq, but only populated, "relevant" areas, according to Kirby.
US commanders have said that the Iraqi army needed to be reorganized and armed before staging a major counter-offensive to roll back the IS from large swathes of territory it seized last year.
The officer overseeing the US-led campaign against the IS group, General Lloyd Austin, head of Central Command, told The Wall Street Journal on Thursday that Iraqi forces would be ready to launch a counter-offensive to recapture the northern city of Mosul by the summer.
Since August 8, there have been nearly 2,000 air strikes in the fight against IS in Iraq and Syria, with American warplanes carrying out more than 1,600 of those raids, defense officials said.