A US serviceman has been killed in northern Iraq, US Defence Secretary Ash Carter has said.
Mr Carter told reporters that the death was the result of enemy fire, but provided no further details.
The US-led coalition against so-called Islamic State said the American had been advising Kurdish Peshmerga troops.
He was killed about 5km (3 miles) behind the Peshmerga's frontline with IS after militants penetrated it using a convoy of truck bombs, it added.
A senior Peshmerga official told the Reuters news agency that the death occurred near Tel Asqof, a town 28km (17 miles) north of Mosul, which was overrun by IS on Tuesday.
Tal Asqof, a predominantly Christian town, was later recaptured by the Peshmerga fighters and Christian militiamen.
The Peshmerga official said early information suggested the soldier was killed by a sniper.
A US official, who did not want to be named, told Reuters that the soldier was killed "by direct fire" from IS
"It is a combat death, of course, and a very sad loss," US Defence Secretary Ash Carter said.
Earlier, there were reports that IS militants had attacked on multiple fronts in northern Iraq overnight.
An Iraqi military source told the BBC that special forces had foiled an attack by five suicide bombers in the village of Khirbirdan, south of the IS-held city of Mosul.
A Peshmerga source said an attack on Wardak, east of Mosul, was also repelled.
Since the US launched a military campaign against IS in Iraq in August 2014, the Peshmerga have driven the group back around Irbil and advanced towards Mosul.
US fighting force
There are more than 5,500 US military personnel in Iraq. Some 3,870 are deployed to advise and assist local forces fighting IS militants.
The remainder includes special operations personnel, logistics workers and troops on temporary rotations.
Last month, the US announced it planned to send 200 additional advisers to Iraq by the end of the year and deploy them closer to the frontlines so that they could assist in the operation to retake Mosul.
The US serviceman whose death was announced on Tuesday is the third to have been killed in Iraq as a result of hostile enemy action since IS took control of large swathes of the country two years ago.
In March, Marine Staff Sgt Louis Cardin was killed in a rocket attack by IS while providing force protection fire support at a coalition firebase near Makhmour, south-west of Mosul, that had only become operational a few days earlier.
Army Master Sgt Joshua Wheeler died in October during a special forces mission to rescue hostages held at an IS prison near Hawija, west of Kirkuk.