"Unknown gunmen attacked a checkpoint located between Haditha and Baiji, killing eight" soldiers, a police major said on condition of anonymity.
They then "collected their weapons and put the corpses in a vehicle and set it on fire" following the attack early today, he added.
The attack on the soldiers, who were members of the 7th army division, took place about 100 kilometres northeast of Haditha, in Anbar province, the officer said.
The checkpoint was on a strategic road linking the two Sunni provinces of Anbar and Salaheddin, and which the insurgents used to transport weapons between the two provinces.
After the creation in Anbar of the Sahwa militia, comprised of Sunni tribesmen who joined forces with the US military against al-Qaeda from late 2006, many insurgents went to Salaheddin.
Qaeda-affiliated militants have claimed responsibility for previous attacks in which security force members have been killed and their corpses burned, though there has not yet been a claim for today's attack.
The violence comes after al-Qaeda's front group in Iraq threatened a campaign of 100 attacks, starting in mid-August, to avenge the death of Osama bin Laden in a US special forces raid in Pakistan in May.
Violence is down across Iraq from its peak in 2006 and 2007, but attacks remain common. A total of 239 people were killed in violence in the country in August, according to official figures.