The attack was one of several reported across southern Afghanistan on Friday and late on Thursday. More than 600 people, including soldiers, international workers and civilians, have died in violence in Afghanistan this year.
"When our forces reached the scene of the explosion, they saw four American soldiers lying on the road covered in blood. They were seriously wounded," said General Salim Khan, deputy police chief of Kandahar province. "I do not know if any of them died."
A policeman at the scene, Tor Jan, and several witnesses also said they saw four injured American soldiers on the road. Tor Jan said two of the soldiers had died, although Khan would not confirm that account.
Varying casualty figures
Master Seargent Ann Bennett, a spokeswoman for the US military in Kabul, confirmed that an explosion had occurred on the road right outside a joint military-civilian reconstruction team headquarters, but said her initial report listed just one soldier injured and one vehicle damaged.
"When our forces reached the scene of the explosion, they saw four American soldiers lying on the road covered in blood. They were seriously wounded"
General Salim Khan, deputy police chief, Kandahar province
The discrepancy has yet to be clarified.
About 60 US soldiers and more than 100 Afghan security forces cordoned off the area, a busy road that leads from the American base to the centre of town, warning journalists not to take pictures.
But the charred remains of one car - presumably the one carrying the bomb - could be seen. It appeared to be a taxi.
Khan said authorities believed the bomb was set off by remote control, as no body was found inside the wreckage.
US soldiers, with search dogs, were frisking at least six men who had been made to sit on the ground with their hands raised in the air.
Kandahar is the main city in southern Afghanistan, and home to a large US base. The region sees frequent attacks, often against Afghan and US troops patrolling its dusty roads.
Afghan forces frequently clash
with suspected Taliban rebels
Meanwhile, suspected Taliban rebels opened fire at Afghan government troops and election workers in two separate attacks in Helmand province, which borders Kandahar to the west.
The attacks sparked shootouts in which four fighters were killed, said Haji Mohammed Wali, a spokesman for the Helmand governor.
The first attack occurred late on Thursday in the district of Mama Karez, as the soldiers patrolled in a pickup truck. Three Taliban soldiers were killed during the one-hour gun battle that ensued.
Later on Thursday, two Taliban fighters riding on motorcycles opened fire on two pickup trucks carrying Afghan election workers in Lashkargah, the capital of Helmand, Wali said.
Security guards accompanying the election officials returned fire, killing one attacker, he said. The election workers were not hurt.
The steady stream of violence by Taliban and al-Qaida fighters has led to concerns that the 9 October vote will not be able to take place in a free and fair atmosphere.