Suspected Taliban also killed two Afghans because they had registered to vote in upcoming elections, police said.
In one incident, 10 to 20 gunmen attacked two vehicles with machine-guns and rocket-propelled grenades 35 miles northwest of Qalat, the capital of Zabul province.
"Four of the militants were reported killed in an ensuing exchange of gunfire and another four were detained," an American military statement said.
The vehicles were damaged but none of the soldiers were hurt, it said.
Elsewhere, two soldiers were hit by their own vehicle after their convoy was attacked by small-arms fire and rocket-propelled grenades 50 miles north of Qalat.
"The convoy was stopped after an RPG hit in front of the lead vehicle. Soldiers in the rear vehicle dismounted and two of them were injured when they were struck by the vehicle from which they dismounted," another statement said.
The two soldiers were flown to the U.S. base in Kandahar for treatment. They were listed in stable condition.
The military didn't make clear if the soldiers involved in either incident were Americans, who make up the bulk of the 20,000-strong U.S.-commanded force here, or from one of the smaller national contingents.
American units have been operating in Zabul, a hotbed of Taliban-led resistance to the presence of foreign troops and efforts by the government of U.S.-backed President Hamid Karzai to extend its influence.
The militants have denounced plans to hold national elections in October, which Karzai is expected to win, and Zabul police reported that suspected Taliban killed two Afghans on Tuesday because they had registered to vote.
The men, whose bodies were found in Thakathoo village, were carrying voter ID cards issued in Qalat, deputy police chief Ghulam Jailani said.
The men were likely killed as a warning to other in the district, known as Arghandab, not to sign up when a voter registration team arrives there in the next few days.
"They want to sow fear," Jailani said.
At least eight election workers have been killed so far this year. Taliban militants were also blamed for killing as many as 13 Afghans in neighboring Uruzgan province last month because they were carrying voter registration cards.