Afghan security personnel stand near a damaged bus that was targeted by a bomb blast in Kabul.

At least four people were killed Monday when a bomb tore through a bus in Kabul, an official said, marking the second day straight that attackers have targeted a bus.

The vehicle was taking workers home from a government watchdog agency ahead of the Eid holidays marking the end of Ramadan when the bomb exploded.

Farid Ahmad, a spokesman for the Independent Administrative Reform and Civil Service Commission (IARCSC), told AFP the bomb had been concealed on a bicycle along the side of the road.

"Unfortunately, we have lost four colleagues, nine have been wounded", Ahmad said.

Interior ministry spokesman Nasrat Rahimi put the toll at five dead and 10 wounded -- all of them civilians.

He said the explosion had been caused by a sticky bomb, a common threat in Kabul where criminals and insurgents slap explosives under vehicles.

No one immediately claimed responsibility.

The blast comes one day after two people were killed and dozens more wounded in Kabul as a wave of bombings hit civilian targets - including a university school bus - across the Afghan capital.

The Islamic State group claimed responsibility for those bombings on Sunday.

Buses carrying government employees have remained a constant target of militant attacks during the rush hour in Kabul.

The IARCSC was established in 2002 with the aim of providing oversight to the notoriously corrupt Afghan civil service.