More than 50 air strikes in recent days have allowed the Kurds to recapture some 100 square kilometers (38 square miles) of ground near Sinjar, said Lieutenant General James Terry, head of the US-led campaign against the IS group.
Kurdish peshmerga forces said earlier they had captured several villages and were rolling back the IS militants around Sinjar in the country's northwest.
It was the seizure of Sinjar by IS jihadists in early August and the plight of the mostly Yazidi minority population there that President Barack Obama cited when he first announced US military intervention in Iraq four months ago.
US and allied aircraft have carried out 1,361 raids against the IS group since bombing began on August 8, Terry told reporters. The strikes were extended into Syria on September 23.
Coalition bombing near Sinjar marked a spike in air raids in Iraq this week, and Kurdish officers told AFP that fresh strikes were carried out on Thursday north of Tall Afar.
The advance of the IS militants, who seized vast swathes of territory in Iraq earlier this year, had been stopped and the group was having difficulty moving and communicating as a result of the air raids, Terry said.
"My assessment is that Daesh has been halted . . .and is attempting to hold what they currently have," he said, using the Arabic acronym for the militant group.
"I think they're having a hard time in terms of communicating right now, in terms of resupply."
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He said Iraqi security forces had cleared parts of Ramadi in the west but the east and northeast sections of the city were "still contested."
Some areas in and around Baiji -- north of Baghdad -- remain the focus of fighting between Iraqi government forces and the militants.
The general said the pace of the air strikes was appropriate at the moment, despite appeals by Iraqi Prime Minister Haider Abadi for more air power and heavy weapons.
"I think we've got it just about right," Terry said. "The key here is building the capability inside the Iraqi security forces, give them an offensive mind set, and we'll continue to strike."
Asked about plans to arm and train Sunni tribes in Anbar province to fight the IS group, he said "the conditions are getting better for that every day."
"I see the current prime minister at least moving in that direction and we'll continue to support them and encourage them to bring the rest of those that want to come to the side of the government of Iraq, support them to bring those around," he said.
In a departure for the US government, the American general used the Arabic acronym "Daesh" when referring to the jihadists who have declared a "caliphate" on seized territory in Syria and Iraq.
Until now, US officials have used an English acronym ISIL, for the so-called "Islamic State" group. Terry said allies in the region had requested Washington use the Arabic acronym, Daesh.
"And I would just say that our partners, at least the ones that I work with, ask us to use that, because they feel that if you use ISIL, that you legitimize a self-declared caliphate, and . . . they feel pretty strongly that we should not be doing that."