The US military said at least four US soldiers were wounded, while police said at least five Americans died.
"The initial report we have indicates an improvised explosive device hit a coalition convoy in Kandahar today," said US military spokeswoman Lieutenant Cindy Moore. "Four US soldiers were hurt and evacuated for treatment."
A Taliban spokesman claimed responsibility for the attack in a call to Aljazeera's correspondent in Islamabad.
The bomber targeted three US vehicles and blew himself up on Monday morning in the Mirwais Mina area, the spokesman said.
Eight US soldiers were killed in the explosion, the spokesman said.
Report of deaths
Earlier, a senior Afghan police officer in Kandahar said at least five US troops were killed in the blast that came from a taxi as a US military convoy was passing along a main road to the west of the city.
The officer, who did not want to be identified, said it appeared to be a suicide attack in which the driver of the taxi died.
"This was a suicide attack," he said. "The person in the car that carried out the act has been torn into pieces. The car approached the American convoy of about 20 vehicles.
"The latest report I have is that five people in the vehicle that was hit by the suicide car have been killed."
A Reuters Television News cameraman near the scene saw a US helicopter evacuating casualties from the site of the blast, in Mirwais Mina, 10km from Kandahar.
The attack comes amid a rise of Taliban-linked violence in southern and eastern Afghanistan since March in which 13 US soldiers had been killed before Monday's attack.
The United States commands an 18,300-strong international force in Afghanistan, most of whom are American, fighting Taliban and al-Qaida members and hunting their leaders, including Osama bin Laden.