US soldiers were seen flying helicopters at the base in Makhmur, 50 kilometres (31 miles) away from Irbil.
This came as a Pentagon spokesman said that more than 100 US troops are embedded with Iraqi forces, including the Kurdish Peshmerga, as they advance toward Mosul.
The spokesman, Navy Captain Jeff Davis, told reporters that the Americans are "well back" from the front lines as they advise the Iraqis and perform other tasks such as relaying information received from frontline Iraqis about potential Islamic State targets for coalition airstrikes.
Some hundreds of other US troops are in support roles such as processing intelligence and providing logistical help from Iraqi staging bases.
Davis said US warplanes are supporting the Iraqis, as expected, and that American artillery is being used against Islamic State targets.
He would not be more specific about the use of US artillery.
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