The suicide attacker detonated his explosives-laden vehicle close to a minibus carrying employees of Afghanistan's intelligence agency, the National Directorate of Security (NDS), officials and witnesses said.

"The number of casualties that police have taken to hospital is four martyred," interior ministry spokesman Zemarai Bashary told AFP.

Health ministry spokesman Ghulam Sakhi Nuroghli said that 29 wounded had been registered by a hospital close to the scene of the blast.

"And one of the wounded is in a critical condition," Nuroghli told AFP.

Bashary would not say if the bomb targeted an NDS vehicle, but an NDS official at the scene said the minibus was carrying its employees to work when it was struck by the blast.

Witnesses reported seeing fragments of a motorcycle and human body parts likely to be from a suicide attacker.

An AFP photographer on the scene said that a minibus and car had been badly damaged in the blast, which took place in front of a psychiatric hospital and close to a mosque just after 8:00 am (0330 GMT)

The last bomb blast in Kabul came on January 4 when a policeman was killed and three people were wounded as the officer tried to defuse a homemade bomb.

The last major attack in the Afghan capital came last month, when two suicide bombers targeted an Afghan army bus, killing five military personnel.

There are about 140,000 international troops in Afghanistan fighting Taliban militants.