Omar Zwak, a spokesman for the governor of the southern Helmand province, says four other soldiers were wounded in Sunday’s attack in the Nad Ali district.
He says the large explosion could be heard from far away, and that the death toll could rise.
The Taliban, who control several districts in Helmand, claimed the attack.
The Afghan government is under pressure on multiple fronts this year as it prepares to hold long-delayed legislative elections even as its security forces struggle to get the upper hand on the battlefield and prevent civilian casualties.
A recent survey found that the militants were active in two-thirds of the country and were fully controlling four percent of it.
