The attack was reportedly carried out by two US helicopters against a checkpoint in the Baraki Barak district of Logar Province on Monday.
Earlier reports had put the number of the Afghan soldiers killed at 14.
"At 6:00 am (0130 GMT) today morning, two US helicopters attacked a checkpoint in Baraki Barak district of Logar province. The checkpoint caught fire," AFP quoted district governor Mohammad Rahim Amin as saying.
Amin said the targeted outpost was "not a suspicious area", adding, "The Afghan flag was waving at the checkpoint in Baraki Barak when the Americans launched their attack."
The US military said it was investigating the matter.
"We are aware of an incident involving US forces in Logar province this morning, July 20th," a US military spokesman, identified as Colonel Brian Tribus, said.