"There was no one injured from our convoy," Johnson said. "Our convoy was on a routine mission."
A spokesman for the Hizb-e Islami armed group of Afghan warlord Gulbuddin Hekmatyar claimed responsibility for the attack.
While allied to the Taliban, Hizb-e Islami has carried out few, if any, suicide attacks previously.
Hekmatyar was one of the chief mujahideen armed men who fought against the 1979-89 Soviet occupation.
He then led his fighters against other factions in a civil war that lasted until the Taliban captured Kabul in 1996.
The Taliban have killed at least 200 civilians in an estimated 140 suicide bombs this year in their campaign to overthrow the pro-Western Afghan government and expel the 50,000 foreign troops from the country.
Violence has surged in Afghanistan this year, with more clashes, roadside bombs, suicide attacks and casualties compared with 2006.