The casualties in Zabul province came on Monday, the same day that two soldiers from the US-led force were killed and two wounded in an attack by Taliban fighters in neighbouring Paktika.
Both provinces are heartlands for Taliban fighters determined to disrupt Afghanistan's first ever direct presidential elections scheduled for 9 October.
"The soldiers were out on a routine security patrol with high mobility multi-wheeled vehicles (Humvees)," a statement from the US military said on Tuesday.
"The two wounded soldiers are pending medical evacuation to Landstuhl, Germany."
It said the fighters were killed the same day in Zabul by an Apache helicopter after they attacked a Black Hawk helicopter with small arms and a rocket. The Black Hawk suffered no damage or casualties, the statement said.
Afghans are being told it is their
religious duty to not to vote
Taliban supporters, meanwhile, have pasted posters on mosque walls in the central province of Uruzgan telling Afghans they had a religious duty to kill government workers and that anyone who voted in the polls would be considered an infidel (non-Muslim), the Afghan Islamic Press reported.
"The murder of anyone working with the government is not just permissible, it is a religious duty," said one of the posters, according to the Pakistan-based news agency.
"Anyone who believes he is a good Muslim should ask his friends and relatives to desert the government," said another.
"If you vote, it is like helping infidels and then there is no difference between you and them."
"Anyone who believes he is a good Muslim should ask his friends and relatives to desert the government"
Uruzgan is in what US-led forces call the "badlands", areas south and east of the capital where the Taliban and their sympathisers are strongest.
More than 17,000 US troops and their allies are fighting the Taliban, who they overthrew in 2001 for supporting Usama bin Ladin and the al-Qaida network.
Interim President Hamid Karzai - in New York to attend the United Nations General Assembly - is expected to defeat 17 opponents standing against him in the election, for which more than 12 million Afghans have registered to vote.