An explosion in the center of the Afghan capital Kabul destroyed a bus carrying employees of one of the country's biggest telecoms firms on Monday, killing at least one person and wounding eight, Interior Ministry spokesman Sediq Sediqqi said.

Security forces surrounded the smoking remains of the bus, which appeared to be completely burned out.

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 Islamic State, but the two attacks underline the broad security threat in Afghanistan, where the Taliban have stepped up their insurgency against the Western-backed government.