Fighters from the al Qaeda linked Islamic State of Iraq and the Levant (ISIL).
A spokesman for the Kurdish peshmerga force said US warplanes on Thursday bombed Islamic State jihadist targets in two areas of northern Iraq.
"F-16s first entered Iraqi airspace on a reconnaissance mission and are now targeting Daash (Islamic State) in Gwer and in the Sinjar region," Holgard Hekmat told AFP around 2100 GMT.
Pentagon spokesman Rear Admiral John Kirby said that reports of US air strikes were false. "No such action taken," he said on Twitter.
The Kurdish official said: "They just struck the bridge linking Mosul to Gwer. The bridge had been used by Daash to channel reinforcements and ammunition to Gwer."
He added: "Their main supply line is now cut, they are isolated in Gwer and the F-16s are striking them there." Hekmat did not elaborate on the targets in the Sinjar area, further west.
Gwer lies just 30 kilometres (20 miles) southeast of the main checkpoint leading to the autonomous Kurdistan region.
A series of jihadist attacks in that area and further east in the Sinjar area near the Syrian border has displaced tens of thousands of civilians in recent days.
Because the areas allegedly hit by the US bombings are controlled by jihadists and have seen mass civilian exodus, AFP was not immediately able to verify the Kurdish claims through witnesses.
Residents on the Kurdish side of the border reported unusual aerial activity and the sound of distant explosions but fighting has been relentless over the past few days and it had been reported Washington may simply be considering food drops.
Attacks by Islamic State fighters in the Sinjar area at the weekend forced thousands of people to run from their homes, many of them from the Yazidi religious minority.
Thousands of them sought refuge in the nearby Sinjar mountains, where many are still stranded with no food nor water, awaiting to be rescued.
But cash-strapped peshmerga, stretched thin over a huge front, and federal forces still reeling from a debacle that saw jihadists sweep across much of Iraq's Sunni heartland two months ago, have been unable to reclaim lost ground and effectively assist civilians.