The convoy came under fire from unknown attackers on Saturday near the town of Spin Boldak on the Pakistani border, US spokesman Lieutenant-Colonel Douglas Lefforge said.
"The coalition forces drove through the kill zone, requested close air support and engaged the enemy forces, killing approximately five enemy and pursuing the remaining forces into the surrounding hills," he said in a statement.
"AH-64 Apaches provided the air support, making several passes on the hill, killing approximately 17-19 more enemy."
Lefforge said there were no coalition casualties in the attack, which happened near the US firebase at Spin Boldak. Two rockets landed near the base on Friday night but caused no casualties, the military said.
Earlier, Afghan officials said US aircraft struck positions of Taliban fighters after they attacked a government checkpoint six kilometres east of Spin Boldak on Saturday night.
Spin Boldak District Commissioner Sayed Fazaldin Agha said two government soldiers were killed in the attack, but witnesses reported seeing four dead government soldiers.
He said five suspected Taliban fighters had been arrested and 25 weapons seized in a joint US-Afghan operation against the Taliban in the area on Sunday.
Speaking by telephone from an unknown location, Taliban official Mullah Abdul Rauf said at least 20 government soldiers were killed in Saturday's fighting, involving 200 guerrillas.
"One of our comrades was also killed," he said. "The Taliban fighters later left the area."
Canadian soldiers assumed control
of the ISAF force on 17 July
The engagements would be the bloodiest in Afghanistan since June, when government forces reported killing 40 Taliban fighters in the Spin Boldak area.
Agha said at least 100 Taliban fighters fled into Pakistan.
Khalid Khan Achakzai, a senior local official with the foreign ministry, said the fighting on Saturday lasted about five hours and US forces sent armoured vehicles in support.
Achakzai said the clash involved at least 75 Taliban fighters led by a former minister, Mullah Abd al-Razzaq, commander Hafiz Abd al-Rahim, and Rauf, a former provincial governor. He said the guerrillas came from the Pakistani side of the border.
Rauf said Taliban fighters had also attacked a US base, but it was unclear how much damage had been inflicted. He said the attacks were planned in a meeting three days ago with the shadowy leader of the Taliban, Mullah Omar.
An official with Pakistan's border security force, Major Shaukat, said Pakistan had beefed up security along its border.
More than 100 Afghan soldiers and civilians have been killed or wounded across the south since the start of the year.
Afghan officials say most of the strikes have been organised by the Taliban and allied militants based in Pakistan, although Islamabad says it is doing its best to seal the border.
The US military, leading a 11,500-strong international coalition, said five coalition soldiers were wounded on Friday and Saturday in attacks elsewhere in Afghanistan.
On Friday, eight government soldiers were killed in the south-eastern province of Khost in a suspected Taliban attack.
Foreign troops have been in Afghanistan since 2001, when a US-led force overthrew the Taliban which had sheltered Osama bin Laden's al Qaeda network. Al Qaeda is blamed for the 11 September attacks on the United States in 2001.