A Taliban spokesman said the group's fighters were responsible for Monday's attack in Uruzgan province, where Dutch Nato peacekeepers are to be based this year.
The US military said in a statement that patrolling US and Afghan forces had come under fire from small arms and rocket-propelled grenades and that warplanes had been deployed to help them beat off the attackers.
A surge of violence has occurred in Afghanistan in the past few months as Nato nations prepare to expand their peacekeeping mission and the US hopes to withdraw about 3000 of its more than 18,000 soldiers in the country.
About 60 US soldiers have been killed in combat in Afghanistan over the past year, the worst period for US forces in the country since they helped oust the Taliban from power in late 2001.
The Dutch parliament voted this month to send about 1400 peacekeepers to Uruzgan, despite widespread public concern about the mission.
Militia unit ambushed
Taliban fighters ambushed a militia unit working with US forces in neighbouring Helmand province on Sunday, killing two militiamen and capturing and later killing another six, the militia's commander said.
About 3300 British soldiers will be based in Helmand from around the middle of the year as part of Nato's plan to expand its peacekeeping force in Afghanistan.
Nato is planning to expand its
Insurgent attacks are common in Helmand and Uruzgan since the Taliban were toppled from power in late 2001. Both are opium poppy-growing regions.
In the eastern Kunar province, an Afghan soldier was killed and five were wounded when their vehicle hit a landmine, and an Afghan solder was wounded by a roadside bomb in the Afghan capital, Kabul, police said.
Since November the violence has included 15 suicide bombings.
Most of insurgent attacks have been in the south and east, near the border with Pakistan.
Hamid Karzai, the Afghan president, says he will raise concern about Taliban fighters operating from Pakistan when he visits Islamabad this week.