0221 GMT July 27 2017
Security forces surrounded the smoking remains of the bus, which appeared to be completely burned out, Reuters reported.
A security official said earlier the blast was caused by a suicide bomber on foot, but Sediqqi said it appeared to have been caused by a roadside bomb.
The explosion, as people were leaving work in a well-to-do area of the city, came less than a week after dozens of people were killed and wounded in an attack on the country's largest military hospital by gunmen dressed in medical uniforms.
Officials are still investigating that assault, which was claimed by Daesh, but the two attacks underline the broad security threat in Afghanistan, where the Taliban have stepped up their attacks against the Western-backed government.
Gunmen disguised as doctors stormed Afghanistan's largest military hospital last Wednesday, killing more than 100 people in a brazen six-hour attack.
The savagery of the assault was characterized by how the assailants stabbed bed-ridden patients, threw grenades into crowded wards and shot people from point-blank range.
Afghan forces, already beset by record casualties, desertions and non-existent "ghost soldiers" on the payroll, have been struggling to beat back militants since US-led NATO troops ended their combat mission in December 2014.