The blast hit the NATO coalition convoy during the morning rush hour in one of the busiest areas of Kabul. Public health officials in the capital said four people were killed and at least 22 wounded, Reuters reported.
There was no immediate word on whether any foreign troops had been hurt in the attack, which hit a group of armored personnel carriers designed to withstand large blasts.
However the blast, which security officials at the scene said was a suicide attack, also destroyed or badly damaged a number of civilian vehicles nearby.
The attack follows a threat by the Taliban to target foreign forces in the spring offensive that it launched last week.
Witnesses said traces of blood and clothing could be seen on the ground at the blast site. Kabul television stations earlier reported that at least three people had been killed.
The heavily armored MRAP (Mine Resistant Ambush Protected) vehicles that coalition forces use to travel in Kabul appeared to have suffered only relatively minor external damage, witnesses said.
There was no immediate comment from the Resolute Support mission in the capital.