An explosion in the centre of the Afghan capital Kabul destroyed a bus carrying government employees on Monday, killing at least one person and wounding eight, Interior Ministry spokesman Sediq Sediqqi said.

Security forces surrounded the smoking remains of the completely burned-out bus that was carrying employees from one of the country's biggest telecoms firms.

A security official said the blast was caused by a suicide bomber on foot, but Sediqqi said it appeared to have been caused by a roadside bomb.

No group has so far claimed responsibility for the assault, but it comes as the Taliban step up attacks even before the official start of the annual spring offensive.

"There has been an explosion against a minibus in Kabul," police spokesman Basir Mujahid told AFP news agency, adding more casualties were feared. "Police are investigating the nature of the explosion."

READ MORE: Taliban kills eight Afghan police in 'insider attack'

Gunmen disguised as doctors stormed Afghanistan's largest military hospital last Wednesday, killing more than 100 people in a brazen six-hour attack, multiple surviving staff and security sources told AFP.

Insiders, including two interns, already positioned inside the facility were among the attackers, the sources said.

The savagery of the assault was characterised by how the assailants stabbed bed-ridden patients, threw grenades into crowded wards, and shot people from point-blank range.

Officials are still investigating that assault, which was claimed by Islamic State of Iraq and the Levant (ISIL/ISIS).

The two attacks underline the broad security threat in Afghanistan, where the Taliban has stepped up its insurgency against the Western-backed government and ISIL has also expanded its reach.