Six US soldiers have been killed in a blast in an explosives-rigged house in Iraq's troubled Diyala province, the US military has said.
Diyala province is the focus of a new offensive
The blast occurred while troops were conducting operations, the military said, without giving further details.
Four other soldiers were reported injured in the blast.
US and Iraqi soldiers have launched a large offensive against al-Qaeda in Diyala and three other provinces north of Baghdad.
Maj Gen Mark Hertling, the commander of US forces in northern Iraq, said on Wednesday that "20 to 30" al-Qaeda fighters had been killed in Diyala since of start of the offensive, codenamed Operation Phantom Phoenix.
In Salahuddin province - which is also a target of the operation - the US military earlier reported the death of another three soldiers on Tuesday.
US commanders have said the offensive is aimed at militants who have regrouped in the northern provinces after being driven from Anbar province and Baghdad.
But Maj Gen Hertling said the indications were that militants had moved north from Diyala before the operation began.
"Operational security in Iraq is a problem," he said, noting that the Iraqi army uses unsecured cell phones and radios.
"I'm sure there is active leaking of communication."
Also on Wednesday, a pair of car bombings around 10 minutes apart damaged two Christian churches in Kirkuk, around 180 miles (290km) north of Baghdad, wounding at least two people.