Two American soldiers were killed while advising Iraqi security troops on a mission against ISIS in a mountainous area of north central Iraq on March 8, the US armed forces said.
The two US soldiers killed were Captain Moises Navas and Gunnery Sergeant Diego Pongo.
Iraqi defence spokesman Yahia Rasool had announced a joint operation against ISIS in Makhmour, south of the Qara Chokh mountains, throughout Sunday and into Monday morning.
Mr Rasool said the ground and air operation by the International Coalition for Operation Inherent Resolve killed dozens of ISIS fighters and destroyed nine tunnels and a training camp.
A senior Iraqi military official told The National that the US soldiers were killed in the Makhmour operation, which involved two airborne missions taking soldiers into the difficult mountainous terrain to "arrest some senior targets".
Noting the difficulty of the mission, General Kenneth McKenzie, commander of US central command, said "the terrain is vertical, it's some of the worst terrain in the world.”
He said the drones responsible for the killings were likely commercial DJI drones that had been weaponsied by ISIS. The operation is currently under review by commanders who are assessing for any flaws in the mission.
The last time a US service member was killed in Iraq was August last year when marine Scott Koppenhafer died in enemy fire during combat operations against ISIS.
American forces briefly suspended their “train and advise” mission against ISIS in January after the US killed Iranian general Qassem Suleimani and Iraqi militia commander Abu Mahdi Al Muhandis in Baghdad.
In February, Nato agreed to expand its anti-ISIS training mission in Iraq, sending military advisers to the country.
Military officials say thousands of ISIS fighters are preparing for the group's resurgence in mountainous and largely lawless areas of northern Iraq, such as Qara Chokh, about 60 kilometres south-east of Mosul.
They say militants are hiding in remote cave and tunnel networks to evade detection near Makhmour, the town they controlled in 2014, using scare tactics to coerce local villagers and capitalising on Iraqis largely ignored by Baghdad and mistreated by Iran-backed militias.
They are operating in conditions similar to those that helped the group to reach its previous territorial gains.
"The numbers we believe right now are 4,000 or 5,000 fighters in those gaps, armed," Lahur Talabany, director of Zanyari, one of Iraqi Kurdistan's two intelligence agencies, said in December last year.
"This is not including the sleeper cells they have in the cities, where the numbers are high.