The soldier "died in weapons accident" in Kandahar province around midday, Major Scott Lundy said on Wednesday.
"Enemy action has been ruled out," he said, without giving further details.
Separately, six other soldiers serving with the Nato-led force in the south were injured on Wednesday when their armorred vehicles collided with a civilian truck, 30km south of Kandahar city, a Nato statement said.
Four of the soldiers were treated for minor injuries and released and the remaining two were stable and being treated for non-life threatening injuries, the alliance said. It had no details on the condition of the truck driver.
The US army said on Wednesday that its soldiers had killed 15 Taliban fighters who attacked a multinational force base in mountainous eastern Afghanistan the day before. Two US soldiers were wounded in the fighting.
In a separate incident, suspected Taliban members have hanged an elderly woman and her son from a tree after accusing them of spying for the government, Afghan officials said.
The 70-year-old woman and her 30-year-old son were killed in the village of Daigh in the southern province of Helmand, Amir Mohammad Akhunzada, the province's deputy governor, said on Wednesday.
Afghan police and army face a
determined foe in the Taliban
The two were are believed to have been killed because members of their family work for the police.
After hanging the pair on Monday, the men threatened to kill anyone who worked for the government, Akhunzada said.
The Taliban have stepped up attacks in southern Afghanistan this year. The government says that more than 900 people have been killed in fighting since May.
The latest killings are apparently part of an attempt by the Taliban to undermine support for Afghanistan's pro-Western government as well as part of the militia's attempt to impose strict Islamic teachings.
In other incidents, British soldiers on Tuesday shot and killed an armed Afghan policeman who was wearing civilian clothes in Musa Qala.
Soldiers saw him outside a military base and mistook him for an insurgent, Britain's defence ministry said.
In northwest Afghanistan, at least four people died after 400 fighters from tribal militias loyal to rival ethnic Uzbek regional commanders Abdul Rashid Dostum and Abdul Malik fought each other.
Meanwhile, in Canberra, John Howard, the Australian prime minister, announced on Wednesday that 150 extra Australian troops would be deployed in Afghanistan, where he said security was at its worst since the fall of the Taliban in late 2001.