The suicide attack took place in Chehel Soton, in the southern part of the capital Kabul. A suicide bomber smashed his car into a bus carrying Afghan army troops during the early morning rush hour.

The bus was reduced to a blackened skeleton. Six army officers and seven civilians died, according to the Afghan defence ministry. Another 17 people were wounded.

The car bombing was the second in two days. A similar attack took place on Tuesday, when a suicide bomber hit a Nato convoy close to Kabul airport, injuring at least 22 but killing no-one. The Taleban have claimed responsibility for the blast on Tuesday.

Gates in Kabul at time of attacks

The two attacks coincided with the US defence secretary's visit to Kabul. Robert Gates was in Afghanistan to assess the efforts against an intensifying insurgency. He also met Afghan president Hamid Karzai, who made an appeal for increased assistance.

"Afghanistan actually was ruled by al-Qaeda and their associates. Afghanistan was liberated by the US and the rest of the international community. So we are very grateful. But if you ask me if we need more, I will never say "no" -- I will keep asking for more," Karzai said.

The Afghan government also told Gates that Afghanistan needs more security trainers and equipment to fight the insurgency. Whereas Gates responded that the US was looking for ways to expedite the delivery of weapons and supplies to Afghanistan. He also called on other NATO partners to dedicate more resources to curbing the insurgency, adding that the task would not be easy.

"Our enemy here is hardened and resilient," he explained. "They will not be going easily. As we were reminded by Osama bin Laden’s message last week, our enemies are counting on us to lose our results. They're counting on us to forget that the security of our people, both European and Americans, is inexorably bound to the stability of Afghanistan."

Deadly 2007

2007 has been the deadliest year in Afghanistan since the Taliban were ousted in 2001.

The number of suicide attacks and roadside bomb explosions has increased tremendously across the country. 140 suicide bombings have been blamed on the Taleban.

Both the Taleban and the US-led foreign troops have stepped up their offensives in the southern part of the country. More than 10,000 people have been killed over the past two years. And as the insurgency continues, hopes of stability and peace in the country are fading.