The men stormed the post in the Khashrud district of Nimroz province in the south of the country, the spokesman, Hakim Latifi, told Reuters by satellite phone.
The governor of Nimroz, Karim Barawi, confirmed the attack but said, as far as he knew, four soldiers had been killed.
"Maybe more than four have been killed, but I know of only four," he said.
The Taliban were forced from power by US-led forces in late 2001 but their fighters have launched guerrilla attacks against US and government forces and aid workers ever since, particularly in the south and east of the country.
Nearly 500 people have been killed since August, raising fears the Taliban and their allies were regrouping in remote areas along the Pakistan border.
100th US troop dies
A soldier whose death in a car accident in Afghanistan was announced on Monday became the 100th US troop to die in the country since the start of a US-led operation in late 2001, according to official figures.
The US Department of Defense website says 99 US troops died in Afghanistan up to 9 January this year. Of these, 30 had died in hostile and 69 in non-hostile situations.
On Monday the US military announced that a soldier, who was not identified, had died following an accident on the outskirts of the capital Kabul.
About 10,500 US-led forces are
deployed in Afghanistan
"Saturday morning, a US soldier died as a result of injuries received Friday night in a vehicle accident southwest of Kabul," Lieutenant Colonel Bryan Hilferty said.
"His death underscores the dangers inherent in Operation Enduring Freedom and our condolences go out to his family."
The US-led force of 10,500 has been in Afghanistan as part of the operation since they ousted the Taliban regime in November 2001.