The bomber used a car in the attack, which happened during the morning rush hour on a road in the southwestern part of the city, the source told Reuters on condition of anonymity.

More than a dozen people were wounded in the attack, the second such suicide bombing in Kabul since Tuesday, he added.

It was not immediately clear if the civilians were travelling in the bus or caught on the road by the blast which also caused gas containers in a nearby shop to explode, witnesses said.

They said ambulances rushed to evacuate the victims. A spokesman for the Taliban said a member of the resurgent Islamic group carried out the attack.

Removed from power in 2001, the Taliban rely heavily on suicide raids and roadside bomb attacks as part of their insurgency against the Afghan government and foreign troops stationed in the country.

The worst suicide attack in Kabul hit an army bus in September and killed 28 army personnel.

On Tuesday, a NATO convoy was hit by a similar attack close to Kabul's international airport. There were no casualties among NATO forces, but 22 Afghans were wounded, according to NATO.

A rising level of violence in the past two years has made it the bloodiest period since the Taliban's ouster and left an increasing number of Afghans frustrated with the lack of progress toward peace.

The attack on Tuesday came during a visit by U.S. Defense Secretary Robert Gates to assess the rising violence. A Taliban spokesman said the militant Islamic group carried out the attack to "welcome" Gates.

Afghanistan's army chief, General Bismillah Khan, asked Gates for more security trainers and equipment to fight the Taliban insurgency.

"The biggest problem is we don't have enough mentors, enough advisers," Khan told Gates on Tuesday after the two toured the Kabul Military Training Centre. "I need your prompt attention on this matter."

A recent poll conducted for three Western television networks showed only 42 pecent of Afghans rated US efforts positively, down from 68 per cent in 2005 and 57 per cent last year.

The poll conducted for ABC News of the United States, Britain's BBC and Germany's ARD network also found rising support for the Taliban fighters. Twenty-three percent of those polled in the troubled southwest said people in their area support the Taliban, triple last's year level.