At least 18 people were feared killed on Tuesday when US-led forces clashed with militiamen near Afghanistan's south-eastern border with Pakistan.
In the last ten months it was the biggest encounter that the US forces were involved in, a military spokesman said.
Militiamen loyal to the Afghanistan government helped the US forces in the battle.
The hostile militiamen were suspected to be those of former Afghan prime minister Gulbuddin Hekmatyar, who describes US troops as 'occupying forces'.
The fighting broke out on Monday near Spin Boldak, 450km from the capital Kabul.