The soldiers were abducted by a large group of militants in several vehicles from a small base in the Ain al-Jahash south of Mosul, and their bodies were found in the area on Saturday night, the sources said on Sunday.
But accounts of when the attack took place varied, with a police major and morgue employee putting it on Saturday night, while an army major general said it had taken place earlier in the week.
The police major said the soldiers had been shot in various parts of their bodies and that their hands had not been bound.
The attack comes after militants killed 12 soldiers and wounded 15 in an April 17 assault on a military base west of Mosul, the capital of Nineveh province.
The province is one of the most consistently violent areas in Iraq.
Militants opposed to the Iraqi government frequently target members of the security forces, but it is rare for such a large number of soldiers to be kidnapped at once, especially from a military position.
The killings come as Iraq suffers a protracted surge in bloodshed, the worst to hit the country since the brutal sectarian fighting that peaked in 2006-2007 and killed tens of thousands of people.
The government has repeatedly blamed the unrest on external factors such as the civil war in neighboring Syria.
But analysts and diplomats say widespread anger in the minority Sunni Arab community over alleged mistreatment at the hands of the Shia-led authorities has also played a role in the violence.
HH/HH