A US military spokesman on Wednesday said an apparently timed explosive device blew up near a special operation forces convoy in Khost province on Tuesday.
It was the second attack this week against American troops in Afghanistan. There were no casualties or damage to the vehicles.
US-led forces toppled the Taliban in 2001. About 19 months later, forces supporting the US-backed government of Hamid Karzai and American troops continue to come under attack.
A US-led force of about 11,500 troops is currently searching for Taliban and suspected al-Qaeda members along the Afghan-Pakistan border.
In other developments, more than 400 Afghan soldiers and police fanned out across a remote southern district on Wednesday in a search for assailants who killed five policemen this week.
Twelve villagers had been apprehended on suspicion of aiding the Taliban but none of the gunmen who launched the bloody Monday night raid in Ghorak district had been caught.
The chief of police in the sparsely populated area, northwest of Kandahar city, was among those killed in the attack.
The latest attacks have raised concerns of a resurgent Taliban regrouping in remote southern mountains and deserts.