A spokesman for the Nato-led International Security Assistance Force (Isaf) said: "Right now we think it may have been a private convoy of private contractors."
He said that Isaf troops might have been attached to the convoy to provide security. One soldier could have been among the casualties but he could not confirm this yet, he added.
The bomber, who died in the attack, planted the explosives in a taxi, the Afghan interior ministry said in a statement.
"I was driving. The car just behind me exploded. It was a terrible explosion," one witness said.
"An Isaf convoy was coming from the opposite direction. An Isaf vehicle was on fire but drove away," he added.
The attack was the fifth suicide bombing in the capital this year but there have been several others around the country, mainly in the south and east.
On Friday, a bomber drove his car into a Nato convoy in Tirin Kot, capital of the central province of Uruzgan, killing five children playing nearby, Mohammad Nabi, a provincial government official, said.
Four adult civilians were also killed and seven wounded, Mohammad Qasim, local police chief, said. The Dutch defence ministry said the attack killed a soldier from the Dutch Army's 42nd Battalion.
Another suicide bomber attacked a convoy in the southern city of Kandahar, wounding at least five civilians, a police official said. A man with explosives strapped to his body approached the convoy and blew himself up, a witness said.