The acknowledgement came as reports of more civilian deaths caused by a fresh air raid by foreign forces emerged on Thursday from the neighboring province of Herat.
Tuesday's air strike was summoned after a coalition convoy came under sustained attack from machine gun and indirect fire from a number of houses adjacent to a road in the Bakwa district of Farah, the U.S. military said.
"The coalition convoy returned fire and called for close air support on the enemy positions. A house was hit; eight civilians were killed, two others injured," it said in a statement late on Wednesday.
"Coalition forces never intentionally target non-combatants, and deeply regret any occurrence such as this where civilians are killed and injured as a result of insurgent activity and actions," it said.
Afghan officials said nine people, all members of the same family were killed in Tuesday's bombing.
In Thursday's raid, at least four men were killed, a spokesman for the regional police command said. Witnesses said 17 people were also wounded and taken to hospital.
The U.S. military said the raid was against "high priority Taliban targets" in Herat, adding two "Taliban leaders" and "significant number of other insurgents were also killed".
In a statement, it said, there was no evidence of civilian casualties.
The issue of civilian casualties is highly sensitive one for the Western-backed government and undermines Afghan support for the presence of foreign forces who are fighting the Taliban-led insurgents in Afghanistan.
There has been a sharp rise in violence in Afghanistan this year, the bloodiest since U.S.-led and Afghan forces overthrew the hardline Taliban in the wake of the September 11, 2001 attacks.
The U.S. military says it is investigating reports by Afghan officials that around 60 civilians were killed in two separate air strikes by U.S.-led coalition forces this month in eastern Afghanistan.
More than 800 civilians have been killed since the start of 2007 in Afghanistan by foreign and Afghan forces, according to Afghan officials and the U.N.
Writing by Sayed Salahuddin; Editing by Valerie Lee