The US military said on Thursday the patrol came under small-arms fire from “extremists” in Paktika province, on the border with Pakistan, the day before.
It did not give the nationality of the soldier who was killed, the latest foreign soldier to die in the bloodiest phase of Afghan violence since 2001.
A British soldier was also killed in a clash in the southern province of Helmand on Wednesday.
The Afghan defence ministry said 12 Taliban and two government soldiers were killed and 21 insurgents captured in other clashes in the south and east.
Violence by Taliban rebels has surged in Afghanistan this year to its worst level since the militants were toppled nearly five years ago.
Most of the bloodshed has been in the south and east.
The US-led coalition has mounted offensives in the south and east in recent weeks to push the Taliban back before a separate Nato peace-keeping force takes over from the US-led force in the south.
The relentless insurgency has grown despite the ongoing establishment of an Afghan police force and army, and the efforts of thousands of foreign troops.
Hamid Karzai, the Afghan president, has said the violence will not be ended by fighting militants on Afghan soil but by going after the sources of the militancy, their training camps and funders outside of the country.
“The situation has deteriorated because we have not addressed the sources of terrorism”
“The situation has deteriorated because we have not addressed the sources of terrorism,” Karzai was quoted in the Financial Times newspaper as saying on Wednesday from Tokyo, where he was attending a disarmament conference.
“The sources of terrorism are where they are trained, where they are financed, where they are equipped, where they are mobilised and where they are motivated,” he said.
The government says the Taliban are trying to unnerve Nato as it embarks on what looks set to be a tough ground mission.
More than 1,200 people, most of them militants, have been killed in Afghanistan since January, among them 63 foreign troops, killed in combat or in accidents while on patrol.
The United States has 23,000 troops in Afghanistan, the highest number since its involvement began in 2001.
The Nato force will soon have about 16,000 soldiers in the country.