Major General Mark Hertling, the US commander in northern Iraq, said: "Operational security in Iraq is a problem.
"I am sure there is active leaking of communication," referring to reports that the Iraqi army uses unsecured cell phones and radios.
US soldiers killed
The US military said on Wednesday that a bomb blast in a house killed six US soldiers during Operation Phantom Phoenix, as the Diyala offensive is named.
Four other soldiers were wounded when the "house-borne improvised explosive device" blew up while they were conducting operations.
The previous day, three other US soldiers were reportedly killed when they were attacked with small-arms fire while conducting operations in neighbouring Salahuddin province.
While violence has declined over the past six months in Baghdad and much of the rest of the country, Diyala remains dangerous.
At least 273 were killed last month in the province, an increase over 213 in June.
Meanwhile, in Kirkuk, 290km north of Baghdad, a pair of double car bombings damaged two Christian churches and wounded two people on Wednesday, according to a senior police officer.
Sarhat Qader said the bombings at the Chaldean church of the Heart of Jesus and the Assyrian church of Mar Afram took place within 10 minutes of each other.
The two churches, about 700 yards apart, were empty at the time of the attacks.
On Sunday, a series of bombings targeted three other churches in the city of Mosul, 360km northwest of Baghdad.
No one was hurt in the Mosul attacks.