American air strikes killed about 26 civilians during a battle in western Afghanistan in early May, the US military said on Friday in a report that calculated a far lower death toll than other estimates.
Failure by US forces to follow their own rules was the "likely" cause of civilian deaths, the report says.
The report accepts that at least 26 civilians died, but acknowledges that the real figure could be much higher.
The Afghan government has said 140 civilians were killed in the strikes.
Washington and Kabul have been at loggerheads for weeks over the number of civilians killed in the incident.
The US Central Command, which oversees operations in Afghanistan, has vowed to change its tactics to reduce civilian deaths.
However it said the air strikes were an "appropriate means to destroy the enemy threat" as it battled Taliban forces in Farah Province on 4 May.
US officials investigated seven strikes on Taliban targets in Farah province on that day, and concluded that three had not complied with military guidelines.
The killings happened when US aircraft were called in to bomb Taliban forces that were fighting US and Afghan ground troops near the villages of Geraani and Ganj Abad.
The civilian deaths were likely to have occured during two B-1 bomber strikes that destroyed buildings where Taliban were believed to be hiding, the report said. Ground troops and the bomber crew could not determine if civilians were also in the buildings.
The air strikes fueled public anger against Western forces in Afghanistan, as the United States embarks on a massive troop surge in a bid to quell the growing Taliban insurgency.
The Afghan estimate of 140 civilian deaths would make the military action the deadliest for Afghan civilians since the start of the 2001 US-led invasion.
Afghan President Hamid Karzai has urged an end to US air strikes in the country, a call rejected by Washington.
US Central Command said it reached its estimate of 26 deaths after interviewing villagers, local officials, aid group workers and US forces involved in the battle, as well as looking at new graves in the area.