Talks fail after the government moves paramilitaries to a top-security jail.
Among the soldiers killed were two junior officers, the spokeswoman, who was not authorised to speak on the record, said.
Acting on intelligence reports warning of a guerrilla assault on the village of La Julia, a mobile army unit from the US-trained Omega Force, was deployed to the area, the spokeswoman said. However, the unit was taken by surprise.
Local media also reported the attack.