Government spokesperson Mohammed Nasiem said four people were also injured in the attack in the southern city of Kandahar.
An eyewitness described the attack to RFE/RL's correspondent.
"The convoy was coming from one side and that car was coming from the other side," the unidentified man said. "I don't know how it happened, if that car hit them from the side or from the back of the convoy, all of them were foreigners [in the convoy]."
The blast came two days after militants used twin suicide car bombs to attack NATO peacekeepers in the capital, Kabul. Authorities blamed Al-Qaeda for those blasts, which killed a German soldier and eight Afghans.
In a separate incident, Interior Ministry spokesperson Yousif Stanikzai says five police officers were killed and two were injured when their vehicle today hit a mine in the southeastern province of Paktika. Stanikzai said police have arrested two suspected Taliban members believed to have been involved in the attack.
Afghanistan's Musical Revival
BACK IN THE GROOVE Decades of war and the Taliban's five-year ban on music took their toll on Afghan classical music. Musicians have been trying to resuscitate the art since the end of Taliban rule. But they face serious economic and artistic challenges -- including the threat of possible attack by Taliban fighters if they perform in provincial areas...(more)