The soldiers were found with their throats cut, and had been stripped of their weapons and uniforms after an armed group ambushed them, said medical and military sources in Kasserine near Mount Chaambi, the site of the attacks.
Four other soldiers were wounded, the sources told AFP.
Earlier, state broadcaster Wataniya 1 had reported "eight soldiers were killed as they exchanged fire with a terrorist group in Mount Chaambi".
The channel said that those killed were "members of an elite unit," without giving further details on the clashes.
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Security forces have been hunting militant Islamists in the rugged Mount Chaambi area near the border with Algeria since December, when it attacked a border post, killing a member of the national guard.
The army intensified its search at the end of April, after landmines planted by the Islamists to protect their base in the border region wounded 16 members of the security forces.
Improvised explosive devices have wounded and killed several other members of the security forces since authorities intensified their hunt for the group in April.
The death of the soldiers comes as Tunisia's Islamist-dominated government faces protests following the killing of an opposition politician on Thursday.
It was the second political assassination since February.
Opposition groups have demanded the resignation of the government and staged nightly protests, particularly in Tunis.
But Prime Minister Ali Larayedh firmly rejected these demands today, calling a general election for December 17.