The commander of the 27th division, Namat Allah Tokhi, confirmed the beheadings on Tuesday.
He said the interpreter and the soldier were beheaded after becoming separated from a patrol of Afghan and US-led foreign troops in the Arghandab district of the southern province of Zabul on Monday night.
But government troops later captured and killed four Taliban guerrillas in the same way.
"They cut off their heads with a knife, so when our forces arrested four Taliban, we cut off their heads too," said Tokhi.
Zabul and adjoining southern provinces have been the scene of bloody clashes between Taliban guerrillas and US-led and government forces since American forces removed the government in late 2001.
Taliban fighters have beheaded government soldiers in the past, but it is the first time government forces in Zabul have admitted doing the same, marking the escalation of a brutal conflict that has claimed more than 800 lives since last August.
The Taliban and their Islamist allies have declared war against US-led forces and consider foreign workers legitimate targets as well as foreign and government soldiers and officials.
The guerrillas have vowed to disrupt elections planned for September, and on Monday a policeman was killed and an Afghan UN worker wounded when a UN election vehicle was attacked in the Kandahar province.
The interim government and the UN have appealed to NATO, which commands a 6400-strong force confined mainly to Kabul, to send troops into the unruly provinces before the elections.
NATO is due to consider the issue at a summit in Istanbul next week.