The blast tore through cars on the main airport road near the US Embassy, leaving them tangled hunks of metal.
A Reuters witness saw the dead body of at least one foreigner in a uniform lying at the scene.
Ambulances rushed to the scene within minutes of the explosion, which came just after 8 am.
Tensions have been building in Afghanistan in the wake of a disputed presidential election that has destabilized the country ahead of the withdrawal of most foreign troops by the end of the year.
Taliban insurgents have been exploiting the uncertainty, launching bombings and attacks on government officials across the country.