The blast, on the western outskirts of the city, happened at the start of the morning rush hour.

"When the bus came, an old man got on, then a woman with two children, then the guy wearing the chador entered, and then a big boom," Amin Gul, a witness, told the Associated Press.

The front of the bus was blown apart by the blast, shop windows were shattered and blood and body parts strewn on the ground.

'Blood, bones and flesh'

"I stepped out of the house and I saw thick, black smoke. When I ran to the blast site, I saw around four civilians, including a woman and a child, who were dead being carried to a car," Atiqullah, a resident of the area, told the AFP news agency.
  
"This was an anti-human act that scars the heart of every Afghan"

Zemarai Bashary, interior ministry spokesman
"I cannot describe the scene - blood, bones and flesh. These poor people in this holy month."

The Taliban vowed to step up its campaign of attacks during Ramadan, there have been six suicide attacks that have caused casualties since the month started in mid-September.

Zemarai Bashary, an interior ministry spokesman, said a policeman had shot and wounded the attacker, but he had still managed to detonate his explosives.

"This was an anti-human act that scars the heart of every Afghan," he said. "I hope the Afghan nation never forgives the perpetrators. I believe God will never forgive them."

Ambulances attended the area which was cordoned off by police.

Visiting the scene of the attack, Fatemi said: "They are animals. They don't have any respect for humanity. We can expect more attacks from them."

Second attack

It was the second suicide bomb attack in Kabul in four days. A similar attack on a bus on Saturday killed 28 soldiers and two civilians. 

"It seems to be a copy of the attack on the army," an intelligence official, who declined to be named, said. 

Hamid Karzai, Afghanistan's president, said the attack was a sign of "savagery" and "evil". He accused those who struck during Ramadan of being ignorant of religious principles.

"The enemies of the people of Afghanistan who commit such crimes will never achieve their sinister goals and will face punishment for their actions," he said in a statement.

Desperate to end the spiralling violence, Karzai offered at the weekend to negotiate with Mullah Mohammad Omar, the fugitive leader of the Taliban.

However, the movement has said it will only talk to the government if foreign troops left Afghanistan, a demand Karzai has rejected.