Media playback is unsupported on your device Media caption The bus was carrying soldiers when the attacker struck

A suicide bomber has blown himself up close to a bus carrying soldiers in the Afghan capital Kabul, killing at least four people, official say.

The Taliban said they carried out the attack in the south-east of the city.

They have stepped up their campaign ahead of the departure of Nato-led combat troops at the end of 2014.

The latest attack is the third by the insurgents in Kabul in just over a week. On 17 January, 21 people died in a gun and bomb attack on a restaurant.

In Sunday's attack, two soldiers on the bus and two civilians on the road died when the suicide bomber detonated his vest, a spokesman for the ministry of defence said.

The BBC's David Loyn in Kabul says government buses carrying security forces and workers to ministries in the capital are a common sight - and an easy target.

Earlier this month, a spokesman told the BBC the militants are "confident of victory" over Nato-led forces and already control large areas of the country.