A suicide bomber has struck an army minibus in the Afghan capital, killing at least two people and injuring two others, the defence ministry has said.

"A suicide bomber on a motorcycle detonated his explosives near an army minibus," ministry spokesman Zahir Azimi told Reuters news agency on Monday.

Kabul police spokesman Hashmatullah Stanikzai told AFP news agency that there were casualties among the soldiers but was unsure of how many.

A Reuters witness at the scene saw ambulances taking casualties to hospital, but it was not immediately clear how many had been injured.  

The explosion took place in the eastern part of the Afghan capital but was powerful enough to be heard across the city.

The Taliban claimed responsibility for the attack.

Armed groups have escalated their attacks against Afghan security forces and government employees ahead of a second round of voting next month to elect a new president.

A second round was forced after the elections last month failed to give any candidate the majority needed to replace outgoing President Hamid Karzai.

The attack comes less than a day after US President Barack Obama made an unannounced visit to Afghanistan to thank US soldiers for their service.

Source: Reuters